I'm going to randomly hazard a guess that DHS was there as April 19th was right around the corner, and on April 15th, two bombs exploded in Boston during the marathon.
Seems like a reasonable deployment given the unknown nature (and much speculated ‘Tax activist’) of the bombings in Boston and the April 19th anniversary of the deaths in Waco.
I didn't check that.. Wanted to confirm the post was genuine so checked before and after to make sure he was from Waco really.. but didn't know anything more than this was a bit off.. and wondered what it means.. I looked and looked in coverage for when this Fire was called in and no luck.
You explanation seems reasonable.. but these guys carry real "Communications" with them.. if this thing was called into 911 system.. and it was.. because Firefighters died on the scene.. in the explosion.. then given the threat.. (that you would have thought someone would have considered.. considering.. Fertilizer Plant..) Why were these guys sitting on their hands and not clearing West at least?
I think you are right, the Waco anniversary is the reason.
The US government actually keeps a calendar of national and international event anniversaries, and even some private security companies do so as well. They do this because it actually helps ward off, or at least respond to potential anniversary events by pre-positioning.
http://www.nctc.gov/site/calendar/index.html