If Titan has a methane atmosphere and seas of methane, then why doesn't some meteoroid / chemical reaction ignite and blow the whole thing?
Titan is one of Saturn's moons. Titan has a dense atmosphere, at about 1.5 bars. It also seems to have lakes of liquid methane.
For a conventional combustion, you would need a good Methane-Oxygen mix. Every combustion is basically an oxidization. Apart from very energetic events, such as SL9 hitting Jupiter, you will need quite a bit of oxygen to get a nice, explosive fireball in Titan's atmosphere.
According to Wikipedia, you need two Oxygen molecules for every Methane molecule you want to burn.