I'm an amateur cat scene investigator, but my only real guess is that it has to do with a bug/glitch with the Stormkeeper talent. The two other speculations that I have is that it's either a weapon enchantment proc/or a GFed item macro'd to the Trinket, but only Lightning Bolt seems to be going off at the time of the kill. Here's a pic of his hotbar annotated:
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Lightning bolt seems to be the only button pressed at the time of the kill and Drums/Trinket was pressed after the first Lightning bolt.