Your boiler has too much water inside it, which is putting excess stress on the internal pipework and safety valves.

DIY fix possible

Check that the external filling loop valves are tightly closedBleed water from a radiator using a bleed key to reduce system pressureMonitor the pressure gauge to ensure it returns to between 1.0 and 1.5 barIf pressure continues to rise on its own, call a Gas Safe engineer