Its because (we suspect that) you have a fault in the wires of the machine that means a small electric current is flowing in the line (the one that sits at 240V nominal voltage) that is discharging to ground (earth). This will occur at all times the machine is connected to the supply, not just when it is 'on'. An RCD trip breaker detects this current (because it looks for the same current flowing back down the neutral wire, and if it doesn't find it, it thinks 'fault!', and it trips. Its called an earth leakage fault.