This is due to a short caused by wires not being tight to the electrical components. Or the thermostat is broke and needs replacing. They are between 20-30 dollars.
Why Do I Have To Keep Pushing Reset Button On Electric Hot Water Heater?
I had a similar problem, my was due to the thermostatic copper wire having got pushed too close to boiler. And it was just a matter of pushing the wire slightly away from it. If your boilers older it might be a bit furred up inside and not letting enough water through pipes, consequently its getting a bit too hot too quickly
