Why did the exhaust valve stick open?

 

The valve relies on the spring to make it close. If the spring fails the valve drops.

Then with an overlapping engine if that is what Harleys are the piston hits it.

 

Maybe related to the water pump failure, could possibly have got hot enough to stick the exhaust valve. Does it have not stock exhaust without being retuned. Guys have kept riding when the pump goes out, and I’ve been there too. Oil pressure dropped and lifters clattered. If it was already running on the hot side I could see a pump failure being enough to cause a problem.