Hi,
at depower of your circuit one phase is hanging on a few milliseconds longer than the other phases creating a
phase imbalance leading to an RCD trip event.
It looks like the RCD is 'C' curve, that is to say the type demanded by most electrical authorities for protection
of domestic consumers. There are other types, I think possibly 'D' or 'M' types which are more tolerant of momentary
imbalance that will not nuisance trip.
Any industrial electrician will tell you what RCDs are required in domestic situations and those which are for industrial
situations in your jurisdiction.
Craig