What would cause the truck to slip out of park and go into reversewhile at a nor on 1995 Dodge Ram 1500

while at a normal idel,was prepairing to jump starta chevy s-10 the Dodge slipped out of park into reverse all by it self. HOW? and why would that happen?

Asked by for the 1995 Dodge Ram 1500
your transmission may be slippin cant exactly tell you from here but i recommend you to have a diagnosis check on your transmission