AFAIK all three US legacy airlines (DL/UA/AA) use contract people to do secondary screenings at the gate at LHR. These security screenings seem random and not related to whether one has TSA pre-check or not. I usually fly AA and I have been selected for these secondary screenings a few times, although I have had TSA pre-check via Global Entry for a long time. It is a pain to go through these screenings, but then we live in a very different era from a generation ago.