Bent Exhaust Valves
 

Purchased 2009 Cooper S with engine issues.
upon inspection all the exhaust valves were bent. Timing chain was not broke or slack.
Engine turns over freely with the head removed and upon inspection I cannot see any other issues.
I will have the head back from the machine shop this weekend with all new valves and guides.
what could have caused this issue? Want to make sure I replace any components that would be deemed necessary.

thanks in advance

I should add this is a low mileage 63,000 miles engine and appears to be very well kept.
carfax report shows 1 owner (female) and dealer maintained.