If it didn't do it before then something has changed (inside the pipe leading to the sewer).
It may be as mortar suggests.
Are you saying it doesn't do it on the main cycle (when the wash has finished, but before the rinse starts). That too has a lot of water to disperse down the sewer.
My guess is that there is a partial blockage quite a long way down the pipe, and it is backing up once a big volume of water goes down there. It could easily be underground, 3-4 metres from the position of the machine
Is the washing-machine adjacent to the sink?
if so you could shove a lot of water into the sink (full) then let it drain out to see if it backs up into the sink when drained - at least that way you don't get loads of water on the floor if it does happen.
If you can establish it is a partial blockage where I am suggesting, you probably won't be able to clear it with sink unblocker as it is too far and too diluted that far down.
You will have to use drain-rods from the manhole outside and shove them back up the 100mm pipe towards the downpipe area where this machine drains into.
Drain-rods are a cheap investment - about the same price as 20 minutes of a plumbers time.
Has mine 30 years now.