This video is from Indonesia but I only have very sketchy background info.
It would appear that a group of youths home-made a large firecracker. They brought it to an open field, presumably to set it up and set it off. As they were fiddling around with it, the cylinder accidentally exploded, causing severe, possibly fatal trauma to the youths.
There are two videos of the incident. First video shows the process of setting the firecracker up. The explosion happens at the 2:33 mark. The field is littered with debris – perhaps this was not the first firecracker they set off:
Second video shows some aftermath, but just as every video from Indonesia, the scene is very chaotic, with way too many people in the way. Lower body trauma to the latest victim loaded onto the truck is pretty significant: