Nuclear power is very a big deal for France, and their companies and military have big business tied up in the uranium mines in Mali & Niger (not big benefit for any of the people living there, of course, but who cares about them anyway?); they've been pressuring ECOWAS and the coup plotters since it began to make nice and smash the Toureg to "stabilize" the situation and keep the money flowing north.
That so-called al-Qai'dah connection the media is focusing on is actually sort of legit; Iyad al-Ghali, the commander of the Ansar al-Din forces, has been a prominent figure in the Toureg resistance movements for at least 20 years, he split from the nationalist movements and formed his own party after securing a line of support from the formerly Algerian-based "al-Qai'dah in the Islamic Magreb" that has fanned out across North & West Africa in the last few years, part of which has come in the form of some big guns (and lots of them) taken out of Libya during the great NATO liberation invasion that has worked out so bloody well for everybody.
The nationalist rebels got a share of the money and guns in exchange for letting the beards operate in their territory, though I suspect they're regretting that right about now. A trio of AQIM's top commanders, Abu Zaid, Mukhtar b'al-Mukhtar and Yahya Abu Al-Hammam, are presently in Timbuktu coordinating with al-Ghali to set up their new base of operations. The nationalist forces are setting up in primarily in Gao and both parties are digging in for a longer fight, which is probably coming fairly soon now that what's left of the Mali military backing the coup is reconciling with Western business and the other ruling elites in the area.