I was at Border's today, looking through one of those Osprey books on the PzKW IV that shows you everything, EVEN PARTS OF THE COMMANDER'S ANATOMY THAT SHOULD FOREVER REMAIN PRIVATE, and there were no straps or seatbelts.
I suspect that a panzer that hit a tree at any apperciable velocity, would find its crew severely fux0red.
So, I dispute any claim that, on the face of it, it is absurd to have tanks blow up when they hit trees. It is likely if a tank hit a tree at speed, most of its crew would be, as they say, hors de combat, if not dead outright.