In other threads I read a few posts from people who are optimistic about Legendarying skills to obtain extra perks (while sticking with a select set of skills for RP reasons), but who also have trouble thinking up RP reasons for why a skill would suddenly shift from expert (100) to bumbling novice (15). This thread is for inventing those reasons.

Offhand I can imagine a few possibilities for why a skill would see a sudden drop in effectiveness:

Physical: Ever take an arrow to the knee? The range of motion required to perform many tasks seems to involve only a few muscles, perhaps, but in the act of trying to do it again we find we relied on more than we thought. An inconvenient injury could well account for a sudden failure to swing your blade with the same power, or why that clever one-handed technique is impossible to reproduce. (The limitation here, of course, is there's no hope of convalescing at the Inn over a fortnight; there's still adventuring to be had, and you'll have to fight through that injury if you want to retrain.)

Psychological: Sometimes you get really, really good at something. And then along comes someone who puts your skills to shame, and it rattles you. You start making dumb, noob-class mistakes. Or maybe there was no encounter with someone better, but you had this moment where you tried to do something you've done a million times before, and--whoosh. Nothing. That strike that should have connected, that arrow you shot three feet to the left of that bandit's head, and you just don't know how you could svck like that... And that failure is insidious, takes a deeper toll than you might think. And it takes months to get your confidence back and return to your former, expert self.

Mystical: Back in Morrowind times, there were curses. Mostly these were of Ash-creature provenance, IIRC, and affected attributes directly. But that shouldn't rule out the possibility of curses from other sources, directed at skills, the practical application of attributes. Ever cross one of the Daedra, or a favored priest of the Aedra? Ever let your gaze linger a bit too long upon the daughter of a soothsayer or an augurer? All those bandits you've slain--have you not considered their widows, their daughters, mothers, might be Wise Women, or Hagravens in training? A curse from any of these might well explain a skill that, without consistent practice, inexplicably falters.

Other ideas welcome!

The universal "Magic" RP backstory for anything.

Done.

In order to become MORE skilled, you have to give up knowledge you have now in order to gain more power later on.

Hm, well, back in Oblivion I did this to a character...Upon leaving the starting dungeon, there's that dock. While standing out on the dock, a bandits arrow struck him, causing him to fall into the river and bonk his head on a rock, effectively granting him amnesia, as well as washing him with the current until he was rescued at a town near the river...Leyawiin, I think it was...

So, if you've got a couple of them skills to set to Legendary, you could have it go that your character suffers an incident, and acquires amnesia, effectively resetting your mental knowledge of your skills, as well as sort of giving you some RP hooks.

With enough training you soon realize how rudimentary your skills truly were after witnessing some phenomenon. Ebony becomes in likeness to Iron, and your blade turns limp. The sands of time are turned once more and you begin anew.

Brain damage...too many hits on the head, too much ale, too many potions.

I'll just have my Khajiit Legendarize Illusion, Sneak, and probably Smithing, for "practice". My RP will be that he took time away from his business to create a household for the misunderstood and "different" of Skyrim (which he will do, through Hearthfire). And he has to get back into peak form. They'll be back at 100 pretty quick, because I know how to level those skills with surprisingly little effort, and his level will be that much higher for it.

No good ones. Sorry.

I actually like the idea of a spoiled potion, or rotten ingredients. I'm not sure it's too much of a stretch to suppose that the positive effects of a properly prepared potion ('fortify' one-handed, say) with 'fresh' ingredients could actually have long-lasting, negative effects if prepared incorrectly, or advlterated ingredients ('ravage' one-handed, so to speak). Nothing insurmountable, but why not?