1993 Nissan Truck Blown Head Gasket

I was driving my turck with no issues at all, I turned the A/C like I've done it before some other times, however this time I notice the temp. Starting going up. Very hot, but not to the point to the red mark, just below. I got home an realized that all my coolant was with my oil in my oil pan. I removed more then two galons of merky stuff when I drained the oil. There was no difference in the way the truck was driving. I'm wondering what cause my head gasket to fail. Could have be the age, the pressure, or a crack head? Normal temp in the engine is less then half on the guage all the time.