A sharp-eyed fan pointed out that the D3 Fallen were bald; a change from D2, when they sported greasy-looking ponytails and top knots. Bashiok sayeth:
Bashiok: Do you want a lore reason or an actual reason?
After the deaths of the Fallen champions Bishibosh, Rakanishu, and Colenzo, the Fallen ceremoniously removed their top knots in their honor. Over the years the remembrance of the old heroes waned, new hair styles emerged, and the majority of the Fallen imps now sport mohawks which symbolize undivided unity and strength.
The real reason for any sort of change is purely artistic.
no way, you just made that up
Bashiok: All lore is just someone sitting around and making stuff up. Granted I’m not one of the writers for the game, and what I stated isn’t going to be held as canon, but there’s also the fact that it doesn’t matter why their hair styles changed.
But I think it’s as good a reason as any.