NPR cites.
Drivers driving in a more risky fashion.
More national weather events.
Repair labor rates & cost of parts are rising rapidly.
I would add more EVs in the auto fleet. A minor damage may require replacement of the battery pack that totals the car.
And perhaps it is because modern vehicles are more complex.?
With new features and new safety constraints entering the picture, a lot of additional ECUs were required in vehicles. For example, to introduce Anti-lock Braking System (ABS), Electronic Stability Control (ESP), and Adaptive Cruise Control(ACC), etc. As more electronic systems were spread throughout the vehicle, the need for interfaces between specific ECUs began to emerge.
Thus when an accident occurs many electronic components are damage in addition to bofy panels.
I must say though that I have not noticed big premium increases. Perhaps it is that I drive older vehicles and self insure the physical damage coverage.