Not sure why you're using the parking brake in your garage...
I've never used the parking brake on any vehicle I've owned and I park on hills a lot living in Cental and SW PA... Maybe I've been lucky, buuuut if the vehicle transmission miraculously shifts by itself with the vehicle off then that's what insurance is for in my opinion...
The Parking brake doesn't prevent wheel slide, which you painfully just found out the hard way...
All that aside, it's not suppose to stay locked once disengaged so it seems like something malfunctioned mechanically if operator error wasn't involved.
I would call your insurance companies (vehicle and homeowners) and let them figure it out...
Oh, one other thing I forgot to mention in my last post. The other reason I use the parking brake in my garage (and whenever I'm parked) is because I step up on the rear tires when drying the roof of my truck. Without the parking brake set, the wheels can turn a little bit each way that makes the footing a bit less stable.