It started out simple. The head of Beirut's airport was deemed to be conduct activities supporting Hezbollah while on the job (that is illegal in Lebanon). Unrelated, the government declared Hezbollah's fiber optic network to be illegal because its role in actions against Israel and exploitation by Syria. The problem is that this was conducted while Lebanon is in a crisis. So all hell has broken loose.
Shia Hezbollah and its allies like Shia Amal and Christian Free Patriotic Movement declared this to be a declaration of war. Labor protests started, roads were blockaded, and fighting began. From what I can tell from press reports, pro-West Sunni Future Movement supporters grabbed guns and started taking shots at Hezbollah. Hezbollah, with training from Iran, has easily brushed aside Future Movements and has announced occupation of Sunni and Shia neighborhoods and taking prisoners.
Both sides do not want all out war but tensions are high. Right now Hezbollah is winning but wants to reach an agreement where status quo ante bellium with extras can be obtained. The pro-West March 14 government is trying to save face while radicals push for war. However, the March 14 government is watching in shock as Lebanon's army refuses to take sides. The only side in fact vowing to openly fight Hezbollah is al Qaeda, hardly the best ally for March 14.
Lebanon is one of the few true democracies of the Middle East. Now thuggish gangs of pro-Iran and pro-Syrian forces threaten to bring it down. Such a shame the world does not seem to care.