im getting my decats fitted this weekend and im pretty sure there should be a bracket that bolts to the brace were your original cat use to on the actual decat pipe...surely if it were the decats being the problem it would go from the inside out,not the outside in??my right flexi is ruined but thats because i scrape it on speed bumps (i always go over them the same angle)..could that be your problem??
Im putting a new (ish) HR y pipe with my decats, so if it falls off monday then you might be onto something