Remember, there has been a lot of building construction work nearby, and the construction company may have thrown away some industrial chemicals into that dumpster. Unfortunately, the chemicals possibly could have leaked from the container and when they mix, the result is what's known as a hypergolic explosion. Given that the dumpster wasn't blown to smithereens (you could still recognize it even in its mangled shape), it's either a small pipe bomb or an unfortunate chemical mixing that caused the explosion.
Except they found a pressure-cooker bomb a few blocks away.
And the dumpster had the sides caved —inward—
3rd device found, 5 blocks away; see blueplum’s posts :
http://www.freerepublic.com/focus/f-bloggers/3470601/posts?page=11#11
http://www.freerepublic.com/focus/f-bloggers/3470601/posts?page=12#12