Supply and demand? Which cabin are you looking at?

If it's economy, then there might be a very large contingent on that flight booked as a group, booked very far in advance and taking up a huge part of the plane. Therefore, making the remaining tickets harder to get and much more expensive.

That's a Saturday, so it might just be weekend demand as well.

I was on a flight last year where Globus probably had 100 pax in economy, although I wasn't in that cabin. I just asked the cabin crew when I noticed all the Globus bags the incoming pax were carrying. That had to put price pressure on the remaining few economy tickets.

I've noticed when checking fares that it's not unusual to see for instance $1200 on the day I'm looking at but $3500 on the day before and the day after. Who knows why. Part of that mysterious BA revenue management thingy.