Basically until 1/1 there was a *circular* unimpeded river of lorries (which burst its banks in December)
What UK has now done is permanently dam the flow inc from GB to NI ..not enough to stop it but enough to constrict the flow a long length down river

It means the lorries can't get around quick enough to pick up the next waiting load. And there isn't an infinite number of lorries/vans to make up for those not able to get around quickly enough.
The same thing will build with GB/EU traffic these coming weeks

(IF enough hauliers/drivers bother to attempt it at all through maybe 4 or 5 times 2020 rates).
On GB/EU the dam, through processes on 1/4 & 1/7, will be built up in both directions.

Normally of course trade deals don't dam, but widen the river channels.. to allow greater flow