Glad your guy is OK.
Check the cab for stress areas, specifically roof sections. If the cab came out of shape, it could cause a pop like that.
The chance that air pressure did this is about one in a trillion. It would require a violent expansion event (explosion) that created more volume than the many openings in the cab would normally allow to expel, and then that volume would have to be sufficient to blow out a window on something designed to be driven hard. That kind of rapid volume expansion would show up as damage elsewhere, too, and I could not imagine a source that wouldn't be real obvious (welding gas canister depressurization, etc.).
But, if the glazing got scratched, and pressure was applied the right (wrong) way due to an impact or deformation of the frame, it would shatter.
I have not seen this in a tractor, but I've seen it in other vehicles and in buildings.
EDIT: one of the times I saw something similar it was new windows that had been etched by water. It was odd, but when we unloaded them from the trucks we had to leave them outside under cover to acclimate. For some reason I never quite understood, bringing them straight into the warehouse after unloading would damage (etch) a few of them from condensation. Invariably some would shatter. The really odd thing was these were windows designed for external use (building windows) and I never heard of them having a problem in freezing winters or searing heat, after install. Their major enemy appeared to be baseballs in school yards.