Combustion gasses entering the coolant. Usually head gasket issues, but other possibilities are a cracked head, or a cracked block, or very very rarely a cracked turbine housing on a water cooled turbo. Obviously very high coolant temps will cause expansion and pressurisation up to the blow off pressure of the rad cap. Temp related issues should be obvious by abnormally fast rises in coolant temp. You should be able to isolate which cylinder the head gasket has blown on quite easily, a worthwhile check if the blow is tiny and might be hard to spot by just looking at the gasket after disassembly, or it will help when looking for a crack. A cracked or warped MKIV head is a scrap head though. An old map sensor and 5 volt power supply, connected to a scope can be used to see pulsation pressures in the coolant with a bit of fiddling, if you like gadgets. Removing a plug one cylinder at a time until the pressurisation ceases is quick and easy, sometimes just pulling a coil pack connector to stop combustion pressures in one cylinder is enough to see where a gasket has blown, or where a crack is. Have fun