Parts fail occasionally. It just happens through use at time with no fault on your part.
Hopefully its just the gasket, I'd get the head checked while you have it off.
If its just a gasket, you'll probably be out a few hundred bucks. The gaskets are cheap, but replacement is somewhat labor intensive compared to many jobs.
Expect significanty more for a new head.
03 Rubi on 35's (Weekend Toy)
11 Ram 2500 - 6.7 Cummins (DD/Tow Rig)
05 Liberty Limited (Wife's): Minor mods, OME/JBA Suspension, 245 70/17 Duratracs