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In White Sand 2, by Brandon Sanderson (the Kindle version, at least), there seems to be an astonishing art shift, unexplained and unannounced, at about page 129. Compare these soft tones:

To these crisp lines:

There may be some other slight art shifts. For example, up until page 136, Kenton’s skin seems to be curiously light compared to the previous volume, which seems especially odd given

his Nightside descent.

But it switches to a darker shade about there:

In a serial work, I’d expect that another artist had taken over, but I’ve never seen this in a completed published graphic novel. What’s going on?

There was indeed a forced change to a new artist. The "forced" word comes from the highest source, the State of the Sanderson. As of now, team Sanderson has been silent as to why this artist change took place. Finally, the third volume is due in June (already available for pre-order at amazon) and the artist is another, new one, Fritz Casas.

As noted in another answer, the proximal reason for the change in art is a change to a new artist. While we can't really know for certain the motivation behind this unusual action, I think we can take a guess.

In essence, Sanderson seems to have felt that his original vision for the book wasn't being carried out by the artist. In particular, he felt like the artist was deviating from his script in such a way as to reinforce racial and ethnic stereotypes.

Ravi: I'm curious how you were the graphic novel. because I have the draft.

Brandon Sanderson: Uh-huh.

Ravi: <You were kind enough to send me that>. You know, you were very descriptive.

Brandon Sanderson: <And it didn't translate well?>

Ravi: Not so much-- although I did see <glimpses of you popping
through>.

Brandon Sanderson: Yeah, so, I focus on the positives. <But there might be some things I might not agree with.>

Ravi: I think that's fair. Of course.

Brandon Sanderson: And I-- what we came up with was the <a list of a few things>... inaudible Like the only one that really bothered
<me> was... where is the big battle?

searches through the graphic novel

<The big splash page> right there. I'm like... <when did this turn
into> white people verse brown people? You've read the book. This
isn't about white versus brown. This is, again, how did we end up with
white versus brown? Why aren't these people wearing armor? These guys
are the ones that are outcasts and these are the high society.

He seems to be referring to this page, among others:

Moreover, as hinted at in the last paragraph ("these guys are the ones that are outcasts and these are the high society"), he felt that the art played into stereotypes of cultured Europeans and primitive dark-skinned desert people, which he says was the opposite of what he was trying to convey (what with the Nightsiders having more technological advancement and seeing the Daysiders as primitive, for instance).

While he also blames himself here, it's not too hard to read between the lines and see that he blames the artist somewhat, particularly in the context of the previous quote.

Brandon Sanderson: Yes, that's part of it. Though I don't think we got in the graphic novel some of the important worldbuilding elements,
such as the armor that melts when sprayed with water, the unique forms
of fighting, and the fact that the people you assume are the advanced
ones (because they live in buildings instead of tents) are actually
far less technologically developed than the ones who live out in the
desert. (Because on this planet, that's the "good" land while the low
sands are the less fertile parts.)

That was a dynamic that was very hard to get across in the book,
though, and I don't know that my skill at the time was up to it. I was
disappointed in the graphic novel once the colors and final art came
back to discover a number of pages that looked like brave Europeans
fighting savage desert people--which was the reverse of what I'd been
trying to accomplish. (But is part of our cultural biases, so I'm not
surprised it was how the artists ended up interpreting it. And I'm to
blame for not reinforcing the idea stronger back when it could have
been changed.)

Indeed, if we look at that image, it's pretty easy to see that. It's been a while since I read the original White Sand novel, but as I recall Kerzta was implied to be industrialized (for Dayside), and having armor with religious markings (of the sort that Ais wears, for instance), not being shirtless, spear-wielding berserkers. And it's fairly easy to see that they're portrayed as having darker skin in this picture, which I don't think was the case in White Sand Prime. They certainly wouldn't have darker skin than Kenton, since he has some Nightside ancestry. The overall effect is very 300-esque, with the sophisticated philosopher-kings of the Diem fighting the non-white barbarians of Kertza, rather than the powerful, religious empire of the Kertza fighting the insolvent Diem living on the fringes.

It seems that Brandon was not happy with this art, laying much of the blame on the artist. It's not hard to see from here why Brandon changed artists.