From the article linked at #20:
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
"Designed to explode when handled." That's the definition of salvage fusing. A fuse doesn't have to be some fancy electronic or mechanical device, but it's a still a fuse, even if it's the design of the overall device that makes it "go off when handled", that is still described as "salvage fusing".
From what I can tell from the descriptions, this sounds like it might be a dry-ice 'bomb' or perhaps something like aluminum, water and drain cleaner, which produces hydrogen gas and would do pretty much the same thing. They aren't really "Designed to explode when handled," nor are they fuzed in the sense that most people mean; they're supposed to explode when the gas pressure gets high enough. Probably the plastic bottle was fatigued enough that handling it was the final thing it needed to blow.
This sounds like some idiot kids were playing around. It would be one thing to do this out in the desert where you could toss it a long distance, let it explode and nobody'd get hurt, but it's really stupid to leave it where someone will find it.
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.