it happened to me too. also - on mine - the auto transmission shifter moved from "drive" to neutral on its own. the reason- at least on my car- was that the chasis crossmember that supports the gearbox was bent because of a rock that i had probably hit. it was replaced (bolts straight to the chasis) and all was well.

2001 sporty. 4 inch lift+ body lift-front lovlells coils+extended forks. reat tj. monroe shocks. full skid plate, auxilary fan. external oil cooler. maxxis 8060 235/75 R15 tires on american racing wheels. trackfinder rear diff lock.
http://www.cardomain.com/ride/3948684/2001-kia-sportage