We didn't start the fire!

So today we went on an afternoon walk with Pete and Kristin in their neighborhood. Their neighborhood has a yearly day when the trash company will take any amount of trash for free (or just the normal price paid per pick-up). So we perused the unwanted items for anything fun.

We came across a fire in the front of someones house where there "trash" would be and asked Pete and Kristin if it was legal to have fires like that to which they replied, "I dunno." We thought it was strange especially since no one was by the fire. So as I ran up to the person's house, a lady driving by was like "Should I call 911?" To which I said, "Ya, that'd probably be good." I got to the door and rang the doorbell twice until they answered, saw the fire, and ran around like mad people. As I came away from their house, their neighbors were just watching the fire grow. One man told me not to run by it as he didn't want it to explode and hurt me. Gee, thanks buddy. The guy's friend asked him if he had a hose and so they got their hose ready as finally the owner of the fire finally got outside and had his hose out too. There was smoke everywhere!! It was crazy man. My question is what was in the trash that made it catch fire and gave no worries to those who threw it out??