causes rcd to trip?

Do you seek to know how an RCD trips or what might be wrong with the dishwasher?

An RCD compares current in the Live and Neutral lines - if there's a difference it assumes the missing current to be running to earth and possibly through a person - it's a safety device to prevent electrocution.

Appliances with heating elements often develop a fault in the element - the live resistance wire touches the outer case causing a short to earth - if there's an RCD in the circuit it will trip.

It's by no means certain but there's a very strong probability that your machine has a defective heating element.

I can say no more...
December 2015