I'm putting money on the fault being an aftermarket cat , we see it all the time , I bet there was nothing wrong with the old cats ,We had a corrolla once & kwik fit fitted a new cat anyway long story short they ended up buying customer an oe one at great expense, I'm shocked at the amount of cat's they replace I would say 90% of my customers that have been there for an exhaust they've tried selling them whole system including cat I think watchdog should get involved I reckon I could give examples all night of fast fit incidents.

Just make sure when you find out it is the cat that the customer get's an oe fitted foc as I seriously doubt there would of been anything wrong with old one I normally write a report out explaining why only oe cat's should be fitted to cars with a post cat sensor that normally does the trick.

sorry for the rant
Matt