I have had this happen to me one time when the HOLD button was on. I was stopped at a traffic light that was a 4-way light. For some reason, the sequence skipped the light that I was waiting for and I had to wait for two full sequences (probably about 5 minutes total). When I got the green light, I tried to proceed, but the parking brake did not release. After trying putting the car in neutral and back into drive, the car still did not move. The cars behind me were honking their horns, but I could not move.
Finally, I turned the engine off, restarted the engine and everything was OK. Turning off the engine shuts off the HOLD feature.
As Phil J said, there must be something in the HOLD program that sets the parking brake after a certain amount of time. My dealer could not give me a definitive explanation on this.
I still use the HOLD feature, but my wife refuses to use it because of what happened.