I think the idea is this:
You have a large volume of clathrates underneath ice or frozen soil.
As things warm, they start to break down and a reservoir of methane gas builds up
at high pressure.
Eventually the pressure reaches the point where it can push aside or lift up or whatever the ice at its weakest point
and it finds a route to the surface.
Now you have a LOT of gas rushing through some kind of hole, a little bit like an oil well blowout and the gas flow erodes the sides of the hole and throws soil or ice into the air and generally starts to make a crater.
Furthermore the escape of all this gas lowers the pressure down where the clathrates are quite suddenly, so the breakdown accelerated greatly, providing still more gas to ruch up through the hole.
So not really an explosion, perhaps more like a blowout, but still fairly violent simply because of the amount of gas and the pressure.
At no point does it combust.