Most likely the valve guide siezed on the valve stem. Unusal to say the least, but quite possible. The valve STEM could have been bent to start with, or a piece of debris got in there. Were the heads reworked? If so, they may not have had enough clearence between the guide and stem.
Another possibility is sommething is stuck between the valve head and the valve seat or the valve seat has came out of the head and got cocked sideways. I would pull the one head first, not the whole engine.

Brian