In the early 2000s, teenage music fans were writing RPF about the bands who played at Warped Tour and other popular rock acts.banned RPF in September 2002, around the time fandom was moving to Live Journal, so while many fans congregated on message boards and archives, others posted (or cross-posted) their bandfic to Live Journal communities like slashypunkboys.
The other thing is that there really wasn't centralization in the form of bandom BNFs.
Initially there was, but when bandom grew so quickly almost overnight, a lot of the early fic writers stepped back/away for various reasons.
In this context, the term is a portmanteau of bandslash and fandom, though it is understood differently in other fan communities. The term's first use is hard to pinpoint, but it first became popular in early 2007 when a new wave of fans became interested in the bands and their surrounding fandom.
The bands that make up Bandom are all interlinking, usually having developed a relationship through touring together, being on the same record label (specifically Fueled by Ramen/Decaydance), or having band members who were previously in other band together.
See this flowchart of Bandom interconnections, created by wistfuljane as part of the comprehensive Subset Bandom Connection primer: Many of the band members have been in bands prior to their current ones.