Mate, it's not random...there's definitely some predictability to it. In general I've noticed there is a pattern with
a.) morning KA departures,
b.) CX's USA-bound flights (as explained well above),
c.) CX's evening LHR departures on 74A (also explained above),
d.) Proximity to lounges for CX flights....especially if departing from the outer concourses (gates 40-71). More often than not CX departs from the 60s, which is on the same side as the Pier lounge. You will see certain airlines (Lufthansa, Air Canada, Thai, KLM) who are almost always parked on the 40s side, while I've noticed UA is almost always parked at low # 60 gates.
e.) SQ's afternoon A380 departure and KE's nighttime A380 departure are virtual locks to get gate 15.
f.) BR seems to get gate 36 or 63 (which are next to each other) for afternoon departures.
g.) EK frequently utilizes the 60 gates.