Ground wire blew off new light fixture
I don't see in the photos what type of plastic connectors you are using, but if they are the typical twist-on wire nuts, then the amount of exposed copper you have is much too long. As mentioned in the comments, it also looks like you've nicked the insulation on the hot (black) and ground (green) wires, right after they are separated out from the outer sheath. If that is true, you should cut it back to remove the damaged part. If that's absolutely impossible, what you should definitely not do (but some people might do this anyway....) is carefully cover the nicked portion with scotch 33 electrical tape (not cheap no-name brand). 

It should be easy enough to figure out where the problem is: on the inside of the junction box, right over the screw mounting hole, it looks charred due to the arcing that happened. If the box is charred, there is going to be an equally charred wire that was on the other side of the arc. 

Also, where it appears the arc occurred (right under a screw mount) could be a hint about what you might have done that caused this (did you drive a screw through there when installing the cover and inadvertently drill through a wire?).