Whoa, that had to be very scary - I probably would have freaked and watched the truck insert itself into the lake = whew, good job saving that!!
I never thought about taking wheel chocks, might not be a bad idea. Truck manual or automatic?
Well I did hit the clutch instead of the brake when I jumped into the truck I had left the door open so that made it easier.
I am thinking it just slipped since I had not set the brake hard, if it had come completely off I think the truck would have been deep in the water in a few seconds, that would have been very very bad!