The evidence from the forum is that it is fairly random. There is no doubt that the security posture of the origin contributes to such decisions, and the flight really needs a airbridge to benefit from transit security bypass... it's not essential, but the bus dropoff gate would need to be sterile from other arriving passengers, escalators and doors changed, plane unloaded, then doors and escalators changed again before any other flight can tip out, and all this faff is easier to do on an individual airbridge gate. We have historical reports on the forum where we had a number of FT members on a succession of flights on the past, coming from a reputable origin, one flight gets to bypass transit security, the next couple from the same origin don't, and then the following one does. Apparently there is a list drawn up by Qatar Airport security of which flights that day have the opportunity to bypass transit security and it's meant to be random to keep everyone on their toes.

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