Likely because he figured out what I wish the other Leafs had also figured out; if EVERYONE'S trying to get Matthews the puck, the other teams are going to key in on him. The most effective way to get Matthews good scoring chances is to get the puck to people who aren't Matthews, so the opponents can't key in on him specifically.
It's also why I argue that it's not Marner who changes how he plays with Matthews; it's Matthews who changes how HE plays. Marner looks to Matthews more when he's on the same line, but that's because Matthews is more dangerous, but he'll set up other guys if they're open. Matthews, OTOH, has a really annoying tendency when playing with Marner to stop trying to handle the puck himself and simply try to get himself into situations where he can shoot, which sucks because Matthews is a good passer when he wants to be. And the two were most dangerous a couple of years ago when they were trying to set each other and Bunting up rather than going to the same Marner-carries,Matthews-shoots gameplan they tried way too often this year.