So what's the deal with Dave's Bro on the Beat Mesa when Jack is on his killing spree?

Bro is one of the most mysterious Guardians for sure, it’s likely he had some inkling about what the Beat Mesa and the Scratch was, and perhaps recognising the threat Jack posed, sort of ‘started’ the Scratch himself as a beacon to let the Session know they had to pack up and leave because shit just got real. Of course you could also get into symbology about how Bro stabs into the record Dave wears and how he was the one to scratch his shirt during their Strife, but that’s getting a bit more Meta.