I had it on the BA1389 at the end of January, people start queueing and they were announcing check in wouldn't start for about 20 minutes - there wasn't anyone checking in for the earlier flight but I don't know if they wanted all the previous bags containered and away to avoid mix ups. As it happened it was only about 5 minutes hence my suspicion about them simply wanting to avoid mixing.

My hand baggage also then failed to have an image taken. Which has also happened to me in T2. It's a quick fix as it just goes through the machine again, the problem is when lots of bags are being secondary screened or having similar issues, it might be a while before they get to your bag to give it its second trip.