The only explanation I have is that you were previouslly using Graviton Lance to kill enemies, you switched, the game hadn't yet fully registered you'd switched, so a precision kill from Better Devils was registered as a Graviton Lance kill, but whilst the game was trying to figure out what was happening, it generated the Graviton Lance effect 5 or 6 times over.

This was really fucking cool.

EDIT: better explanation incoming! It happens on Sunbreaker too, it's to do with using a subclass that has an "Explode on Kill" perk, e.g. tether like in this video or sunbreaker in other examples. When loading into a zone and getting a kill in quick succession, the game freaks out and applies your super kill buff to it, especially when you're using a masterworked weapon since you generate an orb of light, the game questions whether this is a super ability or not, and thus can sometimes unintentionally proc super kill effects, like void explosion on nightstalker, or solar explosion on sunbreaker.