AdvisorJim offers a fascinating analysis.
The timing was a direct result of the Cedar Revolution that freed Hezbollah from Syrian restrictions and on Iranian needs to display that they are the most powerful nation in the Middle East. The Cedar revolution and George Bush forced Syrian to pull out of Lebanon. While this does not mean that Syria is not calling a lot of the moves in Lebanon, Syria was able to abandon its earlier efforts to control and reign in Hezbollah.
At the same time, Iran is looking for ways to express the fact that it is the regional superpower, and that Iran is the logical and natural leader of the Islamic Revolution in the Middle East. There is competition between Iran and al Qaeda for this role, and the Iranians want to make sure that everyone knows that the natural leader of the Islamic Revolution is Iran, not some upstart stateless Sunni Arab organization whose leader has to hide in caves somewhere.
This played into the fact that there is also the shift of the Palestinian movement from the older Fatah (which grew out of secular Pan-Arabism) to the newer Islamic Fundamentalist revolutionary organizations like Hamas and Hezbollah which were effectively kicked off by the Iranian Revolution.
All of these elements were pretty stable until Bush/Cheney invaded and destroyed Iraq, while empowering the Islamic Fundamentalists. The Bush effort to spread democracy into Lebanon did even more to stir the pot, getting Syria out of its previous position of controlling Hezbollah and letting the Iranians ship in the missiles that are now hitting Northern Israel.
So why now? Essentially this is what has resulted when Bush/Cheney decided to follow the advice of the NeoCons in the PNAC and invade the Middle East to empower the spread of western style democracy whether they wanted it there or not. The real cause is the game of political 52-pickup that Bush Cheney set off in the Middle East with the Iraq invasion. It shook up all the stable relationships Â as intended by the NeoCons. However, the NeoCons simply assumed that things would settle down around a set of western-style democracies based on free economic markets that the rest of the world could easily deal with. The latest events have proven that assumption to be totally without any basis.
This is a great deal shorter than advisorjim's Diary, but I cheated. He is offering an explanation of all the current battles in the Middle East, so I just narrowed the issue I was discussing down to the timing of the Hezbollah war and why it's all Bush's fault.