Well I ain't
@Opossum, but I do know my Disney, so I'll pitch in a bit here.
Yeah, there's definitely a trend of twist villains in modern Disney movies.
Thing is, this is more of a trend of
predictable twist villains than just twist villains in general.
A lot of earlier Pixar films (which I'm considering to be Disney for the sake of this argument) have twist villains that actually
work.
They are, for the most part, extremely well executed and make sense in the context of the movie.
Toy Story 2 - Villain has a twist
Monster Inc - Villain has a twist
The Incredibles - Villain has a twist
WALLE - Villain has a twist (in the sense that you wouldn't initially expect a villain in a movie like this)
Up - Villain has a twist (in the sense that you wouldn't initially expect a villain in a movie like this)
Toy Story 3 - Villain has a twist (although it's a bit more obvious here)
I'd also argue that Wreck it Ralph's has a decent twist villain considering who he ends up being revealed as.
However, from Frozen onwards, the twist villains either have little/no reasoning behind them or those motivations are revealed in the
last act. They have no time to develop them as a
villain b/c there's not much movie left to develop them properly.