I've never worked in such a large facility, but I am always sorta kinda really terrified of the myriad venting systems that different plant facilities can have, including the tiny little one that I work at. Just because a pressure gauge is low does not necessarily mean the system is empty.
I'll have to read the draft report when it comes out thoroughly (PDF), but it seems like (as these things always are) a cascade of failures, from design on down to day-to-day operation at the plant. (Is anyone surprised (that in the CSB narrative, which may or not be the real thing) that part of this involved turnover between shifts?)
Here are my previous posts on this particular story.
UPDATE: Added the link to the PDF above, here's a Houston Chronicle story on the report from Lise Olsen, the reporter who wrote the Texas Monthly story linked above.