Looks like loads of people avoiding the airport chaos as well, did due to cancelled flights etc. So took the safe way, which was the ferry, but lots of others had the same idea.
It's not exactly anything new either, last time I went in 2015 it was about 3-4 hours waiting, 2nd day of the summer holidays. We blamed the number of people, as opposed to anything else.
Edit: Seems it was way worse in 2016.
Dover ferry port chaos leads to 14-hour traffic jams
Delays at the Port of Dover have caused up to 14-hour tailbacks on the A20 /M20 with Kent Police warning disruption could last for another two days.www.bbc.co.uk
But..... but........ but....(B word I can't say?)