>But I am wondering if this is a result of heat, detonation, junk head, etc.
Anythings' possible...
If you are looking for a specific reason, they range from common issues such as the use of dissimilar metals used in different engine components, which can't be avoided, to the simple mistake of pouring cold water into an already hot cooling system in an effort to cool it down. Unless you know the complete history of a particular engine, this could have easily occurred in the past.
Cracked cyl. heads can also be traced to localized hot spots in an engine, which typically point to a failure of one or more cooling system components. Try to make sure that all hot spots are addressed, especially if a head gasket failed and was recently replaced. It pays to study head gaskets in an effort to pinpoint why they failed. 'Hot spots' are frequently caused by uneven expansion of engine parts, leaking/plugged/mis-routed hoses, and of course cylinder pre-ignition. I find the spark plugs to be a great source of information when these things happen as well.
If a cyl. head has warped even a small amount, it can cause cracking. Best way to avoid issues, as you may know, is to have the heads checked and decked if needed.