Many possible reasons as stated - most of them mental or lack there of (concentration). But if you want to eliminate one by one I would suggest paying attention to your form at release, my problem is a lack of push/pull which leads to a lack of follow through, a floating anchor, a "shaky" bow hand, etc...if I correct my push/pull dynamic release I seem to cure a lot of ailments.
You may also look at the arrow(s) that's a flyer. Is it the same one more often than not? If so I'd bet the nock is crooked....
Good luck and have fun figurin' it out. If you do figure it out let us know because I can use all the help available.