1994 Mitsubishi Galant why are my valves bent

1994 Mitsubishi Galant mileage: 80,000. I recently had the timing belt replaced. Less than two weeks later my engine dies going down the highway (also the same day as the mechanic smogged it and the same day I took it in to him complaining of excessive engine noise and vibration). He said after looking at the vehicle and test-driving it, nothing appeared to be wrong. I had the car towed to the mechanic. The mechanic found the motor had zero compression and the valves appeared to be bent (he showed me the loose rocker arms and pried on the valves till the rocker arms no longer wobbled I never got to see the head). After he removed the head, there were visible marks on the pistons from two of the valves on each cylinder. My question is what caused this? The car is still at the dealer and he is working on it. He hasn't told me what caused the damage.