Why does headgasket blow?

I interpret your question to be: Why do head gaskets (on an engine) blow-out?

There can be several problems:

The engine head itself might have been tightened incorrectly (misstorqued). This can happen when the engine was first assembled or after a repair. When this happens the pressure on the gasket in not even and not tight enough in places, so the gas pressure of the engine finds the weak spot.

The gasket might be defective. Most gaskets have multiple layers bonded together. If the bonding is not correct or a gasket layer was misscut, again, the gas pressure will find the weak spot.

The engine head or cylinder block could be warped. This can happen when an engine severely overheats. The two components will not bed the gasket properly and the result can be a blown gasket.

The gasket could have simply worn out. They do have a life expectancy and require replacement when the engine ages.