Deadly fight a few cycles back, started over an argument about the crawl, that made Boomer a wanted legend.
A few cycles back a deadly brawl broke out at a waystation, touched off by an argument about the crawl. Boomer, already an outspoken critic and freshly disgraced after his Crawl Con ban, killed someone in it and then disappeared into the ether.
An active warrant for the old man's arrest has been outstanding ever since. Rather than ruin him, the incident made him famous: Carl notes the man became a legend after that, which is presumably part of why Tipid felt safe handing him the 106th Bloody Leeches and why the soldiers idolize him.
The killing that turned Boomer from a loud critic into a wanted legend.