I have had this alately. On two of the last 3 jobs and my own house.
Tripped and blew a capacitor on my generator as well.
On the first job it happened it was the customers keen rcd. I tried it on both neighbours properties and no problem. I got a generator to do that job.
But recently it has cost me. Took it to Les and it wouldn't trip for him. Took it to a well known electric motor repair company and they confirmed he motor was ok.
But my recent problem was tripping when I turned off?
Had a new cable made up. 25m x 4mm core.
But this week it turned on and ran ok, turned off ok but then turn it back on and it tripped. Kept tripping every turn on.
Went to Costa for a chill out coffee. Came back and it turned on and off again but just the once then tripped again.
Left it for 3 hours and it worked once then tripped again.
So got an electrical engineer in who started testing every part and could understand the wiring diagram. He also tested the rcd switch.
That bloody machine just would not trip! So now it appears to have corrected itself. He did find a weak earth on the 3 pin plug.
He went so I set it up again and now working fine and every time.
If yours keeps tripping maybe have your cables changed. Flex doesn't last forever.