Return to harbour with an almighty gale behind them creating that following sea. Add in a shallow sandbar and an ebb tide and there you go...
The risks they were taking is that being underpowered (for those conditions) they could not keep their sterns to the sea, as the waves were travelling faster, pushing their blunt sterns sideways into a "broach" (lying parallel with the wave system) probably the most dangerous situation for any boat apart from material failure. Two of them were nearly flipped over and there was a danger at one time in the video that the left hand boat could have crash landed on the stern of the boat ahead, and there was nothing he could have done about it.
With a lot more grunt, they could have positioned their boats just before the crest and ridden one wave almost all the way.
However, they were idiots just attempting it as Jack said. There is more safety in open water for a boat and they could have ridden it out in relative safety.
I would think that commercial considerations probably dictated the risk, i.e. the fish market and being 1st back with the catch..