Since it's going to be awhile, let the Wild Mass Guessing begin!
I have a theory that OCD is a unicorn trait, not just Twilight Sparkle's trait.
My reasoning for this? Look at Rarity. She exhibits a form of germaphobia, she's obsessed with patterns (look at how she arranged the clouds in the season 3 finale) and she's got a job where OCD can actually be very beneficial. (Same for Twilight Sparkle)
I don't know if Sweetie Belle exhibits the symptoms yet -- it could be adult onset.
Also, this happened while I was writing this post:
(6:40:27 PM) Magus Silveresti: By the way, some sad news
(6:40:36 PM) Magus Silveresti: We're not going to see Season 4 of MLP until winter
(6:40:55 PM) Rusty: What...? :<
(6:41:28 PM) Magus Silveresti: Yeah
(6:42:10 PM) Rusty: So we have to wait 9 months for it? x_x
(6:42:36 PM) Magus Silveresti: ...Yes. Yes, Hasbro is pregnant with MLP X3
(6:42:43 PM) Rusty: *snickers*
(6:42:56 PM) Rusty: Congratulations, it's a new season X3