After Voldemort is defeated at the end of The Deathly Hallows, it's mentioned that all the people under the Imperius Curse were freed.

Does that mean that Voldemort was controlling all the Imperiused? Considering people like Stan Shunpike wouldn't have come into contact with Voldemort til later on, and would likely have to have had the Imperius switched from whomever originally placed the curse on him to Voldemort.

Or is this just one of those greater magic things?

No, the Dark Lord didn’t control them all.

The Dark Lord didn’t cast the Imperius Curse on everyone who was being controlled by it himself. Yaxley put the Imperius Curse on Pius Thicknesse when Thicknesse was the head of the Department of Magical Law Enforcement.

“Once again, all heads turned. Yaxley squared his shoulders.

‘My Lord, I have good news on that score. I have – with difficulty, and after great effort – succeeded in placing an Imperius Curse upon Pius Thicknesse.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)}}

Once Thicknesse was under the Imperius Curse, they planned to use him to Imperius other Ministry officials.

“Yes – my Lord, that is true – but you know, as Head of the Department of Magical Law Enforcement, Thicknesse has regular contact not only with the Minister himself, but also with the Heads of all the other Ministry departments. It will, I think, be easy, now that we have such a high-ranking official under our control, to subjugate the others, and then they can all work together to bring Scrimgeour down.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 1 (The Dark Lord Ascending)}}

Though he was ultimately in charge, the Dark Lord didn’t Imperius everyone himself - he had Death Eaters, as well as those already under the Imperius Curse, cast the Imperius Curse on others for him.

The Death Eaters’ curses likely broke too.

As for why the Imperius Curses suddenly lifted, it may be because as well as the Dark Lord himself being dead, his followers were all either killed, captured, or had fled. That might have been enough to release those they’d controlled as well.

“He must speak to the bereaved, clasp their hands, witness their tears, receive their thanks, hear the news now creeping in from every quarter, as the morning drew on, that the Imperiused up and down the country had come back to themselves, that Death Eaters were fleeing or else being captured, that the innocent of Azkaban were being released at that very moment, and that Kingsley Shacklebolt had been named temporary Minister for Magic …”
_{^{- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)}}

With the Dark Lord being dead, and the Death Eaters being inactive, that may have been enough to break the curses. Those who were Imperiused by others under the Imperius Curse would be released once the original Imperius Curse on whoever Imperiused them broke. After the Dark Lord’s first downfall, when he was ripped from his body but not dead, people were released from their Imperius Curses then as well.

“Some say he died. Codswallop, in my opinion. Dunno if he had enough human left in him to die. Some say he’s still out there, bidin’ his time, like, but I don’ believe it. People who was on his side came back ter ours. Some of ’em came outta kinda trances. Don’ reckon they could’ve done if he was comin’ back.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 4 (The Keeper of the Keys)}}

As the Imperius Curse requires some level of active control from the wizard who cast it, since they have to impose their will on their victim, it’s likely to be easily broken if they’re no longer in a position of strength to be able to impose their will over another’s.

“Moody raised his wand, pointed it at Harry, and said, ‘Imperio.’

It was the most wonderful feeling. Harry felt a floating sensation as every thought and worry in his head was wiped gently away, leaving nothing but a vague, untraceable happiness. He stood there feeling immensely relaxed, only dimly aware of everyone watching him.

And then he heard Mad-Eye Moody’s voice, echoing in some distant chamber of his empty brain: Jump onto the desk … jump onto the desk …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 15 Beauxbatons and Durmstrang)}}

It’s likely that the Death Eaters’ Imperius Curses broke when they were either killed, captured, or fled since they wouldn’t have the level of intention and will needed to maintain it. Keeping a victim under the Imperius Curse requires the caster to be able to control and impose their will over their victim. Otherwise, the victim can break it.

“But Winky didn’t know that I was growing stronger. I was starting to fight my father’s Imperius Curse. There were times when I was almost myself again. There were brief periods when I seemed outside his control. It happened, there, in the Top Box. It was like waking from a deep sleep. I found myself out in public, in the middle of the match, and I saw a wand sticking out of a boy’s pocket in front of me.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 35 (Veritaserum)}}

Therefore, the Death Eaters who lived being captured or fleeing is likely enough to make them unable to continue to control those they had under the Imperius Curse, so the Imperius Curses would break.

I don't have the books with me, but there's this quote-

After Bellatrix's death,

“Voldemort’s fury at the fall of his last, best lieutenant exploded
with the force of a bomb.”- From Harry Potter and the Deathly Hallows

The Death Eaters were few in number. They were overwhelmed in the final battle after Nagini was killed. Bellatrix was the last of the Death Eaters (Excluding the Malfoys) to fall in the battle. Stands to reason that ALL the imperius curses were lifted.

P.S. We do not know whether all curses were lifted, just that they were. Quote says- "The imperiused up and down the country were coming back to themselves". It may be some were never found because nobody knew they were imperiused in the first place. I think this was just JK Rowling trying to wrap up the books with good news about Thickness and Shunpike.