It also requires a very specific mixture of 5-15% in the ambient atmosphere to combust properly.

This means the entire release would not burn immediately. Making a soft explosion, not a hard one seen here.

If methane was released from natural causes, it is impossible for the mixture to be proper through and through.

Ideally, the mixture would vary greatly from 72% to 0.00001%. Leaving a small portion properly mixed for explosive combustion. The rest burns off while turbulence mixes the air with the methane.