Well, it all appears to be fairly straightforward.
Bob Gainey comes to the realization that he's no longer up for the day-to-day grind and challenges of being GM of the Montreal Canadiens, so rather than wait until the off-season to make the move, and play out the string between now and then through the trade deadline, he opts to step down now, assume an advisory position and leave the team in the hands of his right-hand man Pierre Gauthier.
Gainey is nothing if not loyal. By making the move now, at a time when the team needs long-term commitment and direction at the trade deadline, Gainey was likely more able to influence who would assume his chair than if he waited to the off-season. At that point, it's conceivable the organization would have been reviewing their possibilities and one of those options might be to look outside the organization for his replacement.
At the end of the day, though, the bottom line is Gainey simply doesn't want to do the job any more and now Gauthier gets his chance.
Some would suggest it's been morphing that way in the last number of weeks and months anyway, that Gauthier has been taking on more and more responsibility as Gainey came to the realization it was time to move on.
It's unusual timing, to be sure, but Gauthier has been an integral part of Gainey's regime so it may not be such a striking difference in the days and weeks to come.