BA “random security check” and not allowing online checkin

Tried to check in online, got an error message saying I can’t online checkin and have to check in at the airport.

Tweeted them to find out what is going on, I was told it was due to a “random security check”, and that I would have to check in at the airport.

Last time I flew with BA I was bumped off the flight at this time of year, as it was overbooked. With SAA struggling this is probably making flights even fuller. And assuming I don’t get bumped, I will have last pick of the seats.

In 15 years of flying in South Africa I have never experienced or even heard of security screening at the booking stage. Is this even a real thing? Can’t find anything mentioning this on the net.

Regardless of this, why would a security check affect checking in or choosing a seat?