Then why didn't all three explode? I think this was shaking of chemicals causing a container to rupture from gas overpressure. I will SWAG it that it popped when he flung the bag into the dumpster but can't tell from the usual crappy reporting we are subjected to.
"The custodian found the three devices about 9 a.m. in a plastic-type garbage bag, Moss said. When he picked up the bag, one exploded, as it was designed to do when handled."
http://www.wxia.com/news/news_article.aspx?storyid=70306
Could be poor workmanship.
I do suspect this was not ME terrorists, but it wasn't a harmless prank either. You don't leave explosives just laying around ready to go off. Sure you make 'em, but then you set them off, or safe them if you can't get them to go off.
Since the custodian did suffer burns, it had to be some kind of explosive, and probably not TATP, because that actually doesn't produce all that much heat, according the articles I've read recently about it. (Besides the explosion not being energetic enough).