Why is my hot water tank randomly emptying through the overflow pipe?
The tank has a serious problem! Pressure is building in the tank and is released by the over pressure valve on the side of the tank and comes out the overflow pipe. This is not normal operation for this to happen! If over pressure occurs even once, it needs to be investigated! If you allow this to frequently happen you are inviting disaster. Get the tank replaced immediately!

Also if you have an expansion tank, the internal bladder may have erupted.