Everyone has to be stamped in, so they all have to be seen by an immigration officer.

Also France had asked the UK authorities for extra space for its immigration facilities in Dover Port, because from later this year, everyone who isn't an EU citizen will have to have their fingerprints and picture taken when entering the EU. Straightforward at an airport, not so much when you're in a car, or a coach. The French would therefore like a larger facility to process travellers, and yes French immigration is in Dover (UK immigration is in Calais/Dunkirk).

Anyway, the UK refused, so France took back control and gave them a taste of their own medicine - it had been the UK that insisted on strict border checks at external borders when they were a member state.

Ferry delays because of bad weather didn't help.