If they launch 4.0.3a next Tuesday, that gives them one week at the current phase of the elemental invasion, it launches a major change to Azeroth in the middle of a holiday, (which they did with Lich King, but annoyed several people working on Hallow chieves), and they launch a major content patch with potential for clitching before a holiday weekend.

Reasons why I could think they would release it next Tuesday:
-Gives them two weeks to work out bug issues with 4.0.3a before Cata ships
-They feel that 4.0.3a is polished enough for release before Thanksgiving without worrying if it would effect employee workload Thanksgiving weekend
-They don't plan to have a lot of staff outages over that weekend/don't care about potential problems that weekend (knowing Blizz, I think it's unlikely they wouldn't care about employees)
-It could build up excitement for Cata more, prompting more people to want to buy it when it ships
-Player outcray for 4.0.3a is so loud that they release it

Out of the reasons I can think of why they would release it next week versus the week after, it makes more sense to me that they would wait until after Thanksgiving to drop a major content patch. While it's well within Blizz's power to release it next week, with the major reasons being that 4.0.3a is as ready as it will ever need to be by its release or giving Blizz two weeks to work out bugs on live before Cata's release and player outcry, I still think Blizz would look at their employees, appreciate that many would not like to work over the holiday, and play it safe on their workload by waiting until the following Tuesday. Still, I would play it safe and get any "going away" things done before Tuesday.