EWR has had a shortage of reserve 757 pilots most of this month. IAH crews have been covering several of the open trips to Europe. The past few days the reserve coverage was down to one or zero pilots and I've seen some LGW, CDG, EDI trips still not covered on the afternoon of departure.
Not sure if that was part of the problem, but wouldn't surprise me if it was a 'looking for a pilot to fly the trip' issue.
DRW