Far be it for me to question the validity of your assertions, but ....... was that once or twice you flew then ?
Okay, its a while since I did lots of long haul, but either you're someone who needs to be checked thoroughly, you're the statistical tail of the probability curve or your memory is faulty.
It is quite a few times - can't quite remember exactly how many. Had it been once or twice i would put it down to chance, however it's every single time. I'm not claiming that this will be everyone's experience and to be fair it has always been long haul. However I have spoken with others who have had a similar experiences (again long haul and bizarrely more often than not T3). I was making no claims to statistical probability, however the number of times it has occurred to me seems to suggest that it happens quite regularly.
I wouldn't think that they would know I was someone who needs to be checked thoroughly as the line usually stretches from the desks to the entrance door of the immigration hall. Clearly they don't know who you are when you enter and I generally get from the red line at the desk to past the desks in around 30 seconds! Suggests to me that the bulk of the wait is getting to the desk!
The main reason, I guess, is that there is that period of time in the morning when a lot of long hauls come into the airport depositing a huge number of people at the same time. Probably at some point the Border Agency might notice this and perhaps consider scheduling an appropriate number of staff to cover this period. Who knows??