A character with an initially complex and even reasonable motive for his actions is Flanderized into a Flat Character who simply does what is expected, regardless of their original motivation.
This is often someone who starts out with a specific goal, as a Well-Intentioned Extremist or someone out for revenge against the perpetrators of a particular wrong (real or imagined), or someone who is driven to actions by unfortunate circumstances. This can make a very deep and often tragic character, but it's hard for the writers to keep it up, especially in a long-running franchise. Perhaps the author simply has trouble following through on their original idea, other parts of the story got more important, or the creator was making it up as they went along. Perhaps later writers who come on in the middle of a work's run just don't "get" it. Sometimes the writer is trying to give some kind of Aesop that because their original motive was grey, this leads to monochrome quickly. Too quickly. On the other hand, it sometimes is intentional. The character may very well have lost track of what they were fighting for and kept going on in spite of this because they have nothing else left.
This often happens when a story has several antagonists, one of whom is only needed for conflict. Expect the character to become a cartoonishly evil megalomaniac. If this happens to a side character, it is often followed by Flanderization. Perhaps even more strangely, the other characters won't notice such decay and will, at the least, be unsurprised that this character is now a walking cliche. If this happens to a series protagonist, expect an Audience-Alienating Era, gnashing of teeth and shrieks of Ruined FOREVER!
This is, if anything, even more common in stories told out of order, as a writer wishing to give a two-dimensional character more depth will go back and write a backstory exploring the reasons behind his good or bad actions, especially as the character tends to be the protagonist of the backstory.
Comics do this a to a certain degree, as various writers are more or less interested in character depth than another writer. This is common in serial works in general for another reason — usually, a villain becomes popular for what they do as a villain and not for the reason they do it, so when they want to keep using a character, writers have an incentive to discard motivations that might get in the way and put them through Flanderization down to their most essential parts. A particularly common form of this for B-list villains is for them to have a clear and understandable motive the first few times they appear, which eventually decays into "get revenge on the hero for the last time they defeated me" ad infinitum.
As villain motivation isn't always seen as integral to the plot, expect villain examples. Compare The Dark Side Will Make You Forget, where the changing of motives is a form of Character Development. Similarly, It Gets Easier, and He Who Fights Monsters are in-story development of motives. Likewise, this may be purposely invoked in order to create a Self-Disposing Villain.
Compare with: Villain Decay, Plot Tumor, and Flanderization (which is a derailing of how the character acts, not his reasons).
Not to be confused with: Out-of-Character Moment, Unbuilt Trope and Character Development
