CI's drop of IAH is probably due to the low passenger capacities because they are not allowed to pick up passengers from SEA and fly them to IAH, and vice versa. If BR or CI were able to pick up passengers at SEA, I bet lots of Airlines in the US will be losing businesses from SEA-IAH due to poor performance on-board. I always wonder why BR and CI never bothered to apply for TPE-YVR-IAH...

Unlike CI, SQ's connection to IAH is via DME, which is not in the USA and is allowed to drop/pick up passengers at DME. SQ is targeting passenger loads between SIN-DME and IAH-DME while loading some passengers directly between SIN-IAH as the route is probably not as time convenient as SIN-LAX and then transit with an alliance carrier to IAH (or even closer airports in Texas).