They have been doing this for decades. When something bad happens it just fires up again. The Alawites in Tripoli largely support the Assad regime. The Sunni's suffered the same indiscriminate violence by the Syrian regime targeted against civilians as what the regime is doing right now, so there's a lot of sympathy. It was bound to happen.
Also there's frustration about the Lebanese military who would not deploy to the North to stop Syrian violations of Lebanon's territory. Syrian soldiers looked for opposition members and they killed Lebanese civilians. However when some pro-Syrian puppets inside Lebanon started to complain about the smuggling of supplies to the Syrian opposition the army did deploy. Since the majority of Tripoli is with the March 14 alliance (the Lebanese opposition), they are against the Syrian regime.
£5 on the Arab world heating up to such an extent you can see the flames from London....
It always was going to spread beyond Syria this unrest but I can't help but feel that it presents a real chance of complete meltdown in some areas of the middle east when the idea of resistance becomes Sunni vs Shia.