If you were going over 3-4 mph this is normal to prevent damage to the transaxle. If you were at a complete stop then I would take it to the dealer if you can repeat it.
Could you have accidentally bumped the shifter right around pushing the button? That's the sort of clumsy thing I might do. It could put you in neutral if you hit the shifter just after pushing P, if your foot was on the brake (I think).
The answers are already given above, but I will chime in just to support. If you press park and it goes to neutral, you were either going over the parking pawl speed limit (3-4mph) or the brake pedal was not depressed fully. Similar results will happen if you try to go into reverse while in motion.