Could be dieseling.

Take a roll of masking tape into a DARK, NO LIGHT closet. As you PULL tape off the roll, watch closely where the glue releases from the roll. You will see a tiny line of light as the glue releases.

I wonder if something similar could be happening in the gel? The bullet ripped it apart so violently, then it came crashing back together and tried to bounce apart again. Perhaps the flash is a similar phenomenon, release of energy as the bonds are ripped apart?

Total speculation on my part..