a guess, and just a guess on my part....

article I read mentioned methane.  That would make sense. 

A dairy that large... it wouldn't surprise me if they were collecting the methane to generate electricity. 

So, the manure is collected from the barn, and stored in a lagoon - manure storage of some sort. The lagoon has a covering over it, so the methane can be collected.  So, my guess would be to look there to find the source of the explosion.  again, just a guess on my part.