The only way I could see a boresnake beaking off in a bore is if the wrong size is used. Like a .22 in a .177, .35 down a .308, and so on.
Every breakage I have witnessed was the correct caliber bore snake. I am not sure what caused the breakage but my guess is that the bore snake was slowly degraded over time by the bore cleaners that were used, although they are supposed to be solvent safe. Kinda like rubber bands left in the sun for too long.