Why is that a common failure mode in a crash? I can't think of a reason or bug that would lead to the doors locking after a crash.

I think it's a well intentioned safety feature that was never fully thought through. Locking the doors in a crash can prevent a passenger from being ejected from a vehicle. However, if there is no reliable way to unlock the door once the acceleration forces have subsided, you've created a death trap.

Most cars lock as you start driving, I assume the issue is they’re not unlocking when crashed.

Fail-safe designs are more expensive because they require redundancies, fully manual linkages, or just non-centralized control.

The Cybertruck went with daisy chained PoE automotive Ethernet variant. The same cables delivering power to subcomponents handle data. Damage/problems in a single component can not only bring down the network but kill power to all the car's subsystems. It means less wiring in the Cybertruck (and lower production expense) at the cost of durability and fail-safety. Someone looked at TokenRing Ethernet and said "yes that is best".