Reports are that they had been unloaded and were in the cleaning dock. Liquified natural gas is very volatile.
Not true. LNG, Liquified Natural Gas won't even burn. It has to be vaporized first.
LNG is at -260°F, so when it leaks out and is exposed to outside air, it quickly warms up, becomes a vapor and can be ignited.
But before the methane vapor will ignite, it has to be mixed with air down to 15% concentration. By the time you get to a mixture that 85% air, the methane is so light it has risen up away from any leaking vessel on the ground. If it gets below 5% concentration, it can not be ignited either.
Because of these physical properties, it is very safe. However, a vessel that has been unloaded, is going to be full of vapor, not LNG. That is the more dangerous condition.
Is LNG flammable? / Is LNG explosive?
http://www.energy.ca.gov/lng/faq.html#900