Georges St. Pierre and his management team called the UFC after learning of Matt Serra’s back injury. According to Dave Meltzer, they asked if St. Pierre could stand in for Serra at UFC 78.
At the time, the UFC was apparently looking to either Thiago Alves or Josh Koscheck, having already been turned down by Jon Fitch for numerous reasons, including prior obligations and difficulties he would face making weight at such short notice. Alves was likely to be offered up to Hughes before St. Pierre stepped in.
The UFC figured that St. Pierre was unapproachable due to a guaranteed title shot he’d already secured. The fight was set to take place sometime in April in his hometown of Montreal, Canada.
St. Pierre however, was apparently getting stir-crazy without a fight lined up for what would have been seven months. There was even talk of him competing in amature wrestling tournaments. That apparently factored into his decision to push for the fight.
Hughes agreed to the fight afterwards, for many reasons, chief among them is likely the fact that the fight would be for five rounds, being an interim title fight, and knowing that St. Pierre will have only four weeks to train, which is half as long as usual.