Electricity is like water, it goes to the path of least resistance. The spark should want to flow to the plugs into the block where the ground is, looks like it had trouble finding an easy path.
I would be checking my plug wires, and engine grounds. If you know somebody that can use an ignition scope, after you get it running again, have them check it out. A scope is the only way to watch an ignition system work in real time under a load. Kind of a dying art, but nothing is better to diagnose ignition problems.