I would imagine snagging it on things, having it drop down due to a big impact and getting tangled in your pedals, or potential impact with your heels since feet are more active on MTBs.
Change the springs if they fall down. I've hit trees and bent frames and not had the kickstand come out. If your heels are flying all over the place, the problem isn't the stand, it's not keeping your feet on the pedals. Heels shouldn't be going side to side when you're mountina biking.