Emergency Brake Releasing

[Crazy Canuck]

The other day I backed my boat down the driveway and stopped on a partial incline to let me 3yo out since he had to pee lol.  I put the truck in neutral (with service brakes on), set the parking brake and put the truck in Park.  I let the truck idle since it was about a 45 minute tow and went around the other side to get him out of the car seat.  He was standing beside me when I reached across the passenger seat to turn the truck off.  When I hit the button, the truck lurched backward slightly to rest on the parking pawl.  It seems that the e-brake released which is confirmed when I went in and restarted the truck.  It was unsettling to have the truck move with my boat attached while standing outside of it.  Scared the young lad as well.  Anyone else see or hear of this?  I have not so far been able to reproduce it which will waste my time brining it to the dealer.  '22 2500 Denali HD/Duramax.