I had a slight 'scare' at INV whilst doing a b2b in Sept, for one of the INV-USA tickets. Walked round from the incoming flight, to find a line of 20+ stretching back from the queuing area for Security, with another 20+ inside. Joined the line, then spotted a chap walk along the right hand side of the line, towards a 2nd gap in the barrier/fence by the wall, and enter. Looked closer to see a small 'priority' sign with BA logo. Was in J, so followed him through, but otherwise would have really struggled, and maybe had to push in (hate doing that). Very easy to miss that 2nd entrance, and for all my poring through posts on b2b's/turnarounds, can't remember any references to it.
As for why there was the queue - wasn't really there long enough to ask, but there was a KLM and an EasyJet kind of around same time (11.00). Also wondered if it might have been a coachload on a group trip. Worth being aware that if it does happen to be busier than normal, a 20-30 minute delay is possible, even there.