I'm going to say that for the 2x4, didn't look like the wood was in the best of condition. But also I don't see how it could not be constrained at the floor - it clearly (based on the position of the shelves after removal) had a lot of compressive forces on it. Would be hard to push it to the side due to friction with the floor - and if it bent only at the top (say no friction from the bottom), then the effective lengthening of the wood (due to the flat bottom) would definitely create enormous friction, since there would be no room for lengthening.
Short story, no mysteries there. Not clear that it would have to be moving all that fast to break the wood if it were brittle. It would just have to deform the wood.
If Continuous press is OFF, then it is possible to initiate Summon from inside the car with two Park presses, and it will take off after you close the door (you have to be very confident to use this mode I think!).
Wood is in good shape. You re right about the compressive forces, it was a bi*** getting a new support in place before putting the car away last night.