Christian McArdle wrote:
The double pole switch might be causing the problem, if both contacts
do not disengage at the same time then this could cause a momentary
imbalance of current.


So how would that work then?

Actually it might work like this:

The pump motor is just a mains coil embedded in plastic. and say this
insulation is going and water getting in to the coil at the neutral end.
While both live and neutral are connected the current flow from neutral
to earth is low enough not to trip the RCD but if the neutral pole of
the switch opens first you momentarily have a flow from live through the
coil (negligible resistance) through the dodgy insulation to earth, so
the RCD trips.