Answer:
There really isn't much in the Bible itself about "fallen angels" so it's not possible to answer this without using other sources which may or may not be accurate (and may disagree with each other).
The general consensus is that they willingly made a decision that led to their "fall" (just as humans did), but that does leave open the question of whether they knew ahead of time what the consequences would be. It's arguable that Adam and Eve, for example, did not, and if they didn't, it's at least possible that the "fallen angels" might not have either.