I've found that it can happen with almost any heavy weapon. (I don't use ESGs often enough, but I've noticed it with every other heavy except Drones) The only thing I can think of is either a lag spike (on single player?) at just the right time to fire the weapon, but not label it as discharged, or that a block of code gets skipped accidentally. As I do believe that that is the cause for the Hand, I'm more inclined to believe the latter.
For those that aren't aware, I believe there is a glitch in the section of code that governs destroying fighters, PFs, drones and shuttles upon the destruction of the carrier, and that is the cause of the spontaneous combustion glitch known as the Hand of Bethke. It would be interesting to see if we could eliminate that section of code all together. Just make the drones they fired lose tracking ability (set their target to null, as happens when the target is destroyed or otherwise removed from the board, and make the enemy destroy the rest of the carrier's "Children".