Loose rocker or the bent pushrod started this, when the valve stem mushroomed it saved your engine. I would guess the piston will have some love taps on it but likely be ok. If the truck is a nice one , please fix it, a good head rebuild with new springs and keepers, new guides will almost guarantee you many years of trouble free miles.
There is no cut and dried reason this happened. You can coulda, shoulda ,woulda all day long and not get the right answer. Likey abuse by a previous owner.
JM2CW.
Just for grins, see if that rocker bolt is streched, if so it got hammered loose, if not it loosened and started this problem.
Ive done so many failure analysis on engine/bearing fails I cant remember them all.
Oh and the valve springs look too clean for really bad guides, when loose they cook the exhausts black.
I was able to remove the rocker arm bolt with just my fingers...not sure at what point this happened.
The truck is pretty nice and I have done a bunch of pretty big repairs on it, I'm just not sure I am up to this repair. Anyone in the Central VA area want to make some money pulling the head on this thing?