Collasped valve lifters??

After hearing loud ticking noises when starting the engine cold I took the car to the dealer... They tell me they found 3 collasped lifters, so they replaced all 12 lifters... Also, due to the top motor tear down, both the UIM and LIM gaskets were replaced... The dealer told me they don't know why I had bad lifters... There was no evidence of coolant in the oil, oil level and pressure was good and the oil only had 1000 miles at the time... It's a 2001 LS, 3.8L motor with 69,000 miles... What would cause lifters to collaspe??