2 big TCXs from Cancun, the EK from Dubai, the LX from Zurich and the LH from Frankfurt all landed in the space of half an hour. It was over 1000 passengers, and some flights were slightly late (including yours) which probably led to UKBA being a little overwhelmed - although all flights arrived within 20 mins of STAs.
Higher proportions of non-EU pax ahead of you will always mean it takes longer to be processed.
Just unlucky, I'd say.