Dunno. Any chance the air pressure was down a bit in the rear, and you hit a pothole or something?
For some reason, vandalism doesn't jump out at me.
Everytime I've had a tire slashed on a car, they've always done the sidewall. Of course, it's easier to get to, and ruins the tire.
You don't happen to park the bike where a bunch of rug rats hang out?
(Apartment complex, or something.)
I can picture some dumb kid doing that....
How much tread life is left? May be worth getting rid of that rear early.....