The valve bent from contact with the piston. I'm guessing you had an overheating problem which can cause all sorts of issues including blowing the head gasket... or afte rthe gasket blew the heads weren't being cooled properly. The overehating of the heads may have damaged the valve stem seals causing the valve to stick and not retract fully. The piston would then impact the valve. The valves are at an angle in relationship to the piston so almost any contact with the piston will bend them. Once it's bent, it won't be able to retract.