Hard to say. Expansion tanks get brittle with age and with a consistent 20 psi (2 bar) of pressure. It can just give way. At the same time, if exhaust pressure got in the coolant flow, that can cause excessive failure. Since it's easy and cheap, you might have mechanics test for presence of exhaust in coolant, just to rule it out. If okay, I would replace cooling parts, bleed the system and carry on.

But 30 mins of up/down temps? - At the first sign of coolant temp irregularities, you should have stopped and investigated, particularly when pulling a load.