I'll just copy what I sent to riot as PM :D (no, I'm not lazy... 
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Hi dear Riot! I just wanted to talk you about a little bug I think Fiddlesticks is having. The thing is that when I was doing Fiddle's dance (Ctrl + 3) he freezed after a short period of dancing. I know this may be a ridiculous thing, but I'd like your game to have no imperfections, so I think you guys would like to take a look on that
It's done on purpose. After a little while of standing still (including dancing, emotes), he will freeze and go into scarecrow mode.
He had an update in an earlier patch, where if standing still for 1.4 seconds, he freezes to be like an actual scarecrow. This also applies to the dance. Hope I could help.
This is actually not a bug. Riot added a feature a couple patches back for Fiddlesticks so that if he doesn't take any action for a certain period of time, he freezes and becomes a scarecrow.
Riot could make it so that this doesn't happen in the middle of his dance, though, so that it won't interrupt it.
