I'm imagining something like this: 1) frozen products of decomposition, including methane, are kept homogenous by temperatures permanently below the freezing point of water 2) Global warming causes a freeze-thaw cycle that briefly melts the permafrost. Methane, no longer trapped by frozen water, rises - but not all the way to the surface because half of a freeze/thaw cycle is freezing. 3) Over time, the freeze-thaw cycle "kneads" methane out of homogeneity and into pockets - just as many small bubbles collapse into few large ones, the "foamyness" of methane in permafrost collapses into cavitation 4) For select large, dramatic regions of deep permafrost and aggressive climactic change, those large pockets eventually reach a critical mass that becomes self-sustaining, ejecting large quantities of gas all at once. See also: Lake Nyos aka "The Lake Nyos Disaster"