One of the main causes of rhabdo in the civvie world is massive crush injuries like you'd see at a nasty car crash or construction site accident. Crossfit is legendary for reproducing these results through overtraining. There are varying degrees of rhabdo, and Stiverne may have been suffering from a minor case of it due to severe overtraining, but if it was really that bad a week out he would never have made it to the fight at all.
12 rounds of getting your shit kicked while trying to walk him down (sort of) might well have caused it, but I haven't seen any conclusive proof that he suffered anything other than a loss of his championship due to being unable to deal with Wilder's movement, length, and power.
Until they publish the medical reports showing a conclusive diagnosis of Rhabdomyolysis, you have to assume it's just a bunch of bs from Bermane's camp as they try to make excuses for their guy and justify a money making rematch.