In a series of flashbacks, Hex reveals that when Welman activated his Gateway command shortly before Bob arrived on Mainframe, it brought the Super Virus Gigabyte to Mainframe's Twin City. This caused an explosion which nullified Mainframe's Twin City and everyone in it, including Welman, and split Gigabyte into Megabyte and Hexadecimal.
Hex explains in a series of flashbacks: when Welman activated his Gateway command near Bob's arrival on Mainframe, the Super Virus Gigabyte was transported to Twin City. This led to an explosion that eradicated Twin City and everyone within it, including Welman, splitting Gigabyte into Megabyte and Hexadecimal.