Has anyone had a horse just start pulling back out of nowhere? And if so, how did you fix it?

Currently stumped.

I'm hanging out. I've never been able to cure one that pulls back.

I never found a way to fix it but the only way I could stop mine was with a gut rope.  He only pulled back when tied to the trailer. He might stand tied all day or 2 mins and then flip out.  The gut rope was the only thing that stopped him from pulling back.

Could never find anything that really stops them. I tie some bale string to my trailer now. It seems as though once they realize they aren't tied solid to the trailer they don't pull back near as often.

Make sure they're not hurting, we had a mare that never hung back, she did it a few times in a row, got her adjusted and she hasn't done it since. 

If that isn't the issue, try a belly rope. I've got one that will randomly do it (he's not sore just an ass), a belly rope cures him real quick.