again check the fluid.. if it isn't full or isn't warm it will roll back.. not enough pressure to hold it. automatics are made to slip a little... the stall converter in them doesn't engage the gears until you rev the engine to a certain preset rpm... (this depends on the cam/stall set you have) like i said they are made to slip til that rpm, so in that case it will roll back. all automatics with high stalls will, and with low fluid or cold. the heat matters cause the fluid expands. that is why it says on the dip stick to check tranny fluid hot and engine oil cold(cause it has to drip back down) got it?
if an automatic didn't slip... when you were stopped at a light and "in gear"(drive) with your foot on the brake the car would stall out like not putting your foot on the clutch at stop. think of the stall converer as you putting your foot on the clutch til you rev the engine to take off...