For those who didn't notice, Isha stole the hextech energy cell from Vi's gauntlets when she saw Jinx pop out hers from her pistol.
Jinx then made Isha steal her gun back from the commune so she could fight off Rictus.
But when she dropped her pistol in the final battle, Isha used her hextech energy cell she stole from Vi to overload the pistol which already contained Jinx's original cell, and that's why it resulted in a massive explosion killing Isha.