Some conference expansions don't work well. The Mid-American Conference had Massachusetts as a football-only member, likely looking to play off of UMass' basketball prestige to get attention for the conference in the Northeast corridor inhabited by a lot of movers and shakers.
It didn't work. UMass has turned into a bottom-feeder in the conference, going 1-11 in both '12 and '13 as it moved up out of FCS status.
The MAC then made an offer UMass could refuse; join the MAC for all sports or get outa Dodge. While MAC football is a step up from the Patriot League for football, MAC basketball is a step down from their Atlantic 10 digs. Note the 6 A10 teams in March Madness, including Dayton making the Sweet 16, to Western Michigan's lone MAC bid.
Thus, UMass will be out of the MAC after the 2015 season. They'll have to figure out what to do with their anemic football program; reverting to I-AA/FCS seems to be a more viable option than trying to migrate over to the American Athletic for all sports, for I'm not sure the AAC would be interested in a football-only UMass.
Being an independent FBS team has few advantages and more expenses, not to mention the problems of scheduling 12 games and arranging bowl ties (if you could actually win six games someday) without a conference tie-in, so I don't see that as a viable option.
It also streamlines the MAC, giving it 12 teams, allowing for a symmetrical schedule and keeping the membership within roadtrip distance of a Great Lake.