There were a lot of people expecting others to pass in corners, and no lifting off out of corners to let faster cars through that were slower down the straights, as well as no blue flags on the day at all. It led to a few issues with multiple cars trying to pass the one at the front who kept flooring it down the straights but couldn't go round a corner faster than a reliant robin.
It was basically down to lots of people not knowing how overtaking on track days is meant to be. Maybe the briefings need to change to make it clearer - after the event on facebook when we were discussing it, there were quite a few saying 'why didn't people pass under braking or in the corners?'

admittedly some from outside the UK where rules differ, but still plenty who were UK based.
The time before that was an MSV day and the morning was kind of the opposite - race teams using it as a test and they got told off (eventually) so it calmed down after the 5 red flags in the morning.