I know ours at times will roll an easy foot or so when put into park. Scared the bajeezus out of me the first couple of times.
If yours rolled multiple feet, only think I can think of is that the park pawl didn't drop down and lock. Might of hung up momentarily.
You'd be amazed at how tiny that little piece of metal is that holds the jeep from moving. I've seen them bent/broken on occasion, usually from someone slamming the vehicle into park while said vehicle is still moving.
If it starts doing it, two choices. Start using the parking brake, or yank the tranny.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.