Back in the 80's, if you had gone to a doctor with a torn pec, the first thing he would've asked you was "So, which steroid cocktail are you on?"
It's not an easy thing to tear a pectoral muscle, even for body builders. But torn pecs were quite common among users of 'old school' steroids. The correlation percentages made it a near certainty that one was associated with the other. The new decade brought with it new steroid usage and, primarily, HGH, which greatly reduced such injuries.
Assuming his injury wasn't incorrectly reported, it sounds like Dumervil didn't get the message that his Dianabol-Winstrol stack is outdated.