Simple answer. Coal bed methane.
Yes, you have the correct answer. Back in the late 1980's I coauthored a paper for the Society of Petroluem Engineers that identified the several causes of the problem. When water is removed from a water-bearing coal bed the methane is released both to the gas well and to any other penetrations, natural or man made. Natural include vertical fractures and permeable bedding planes that may connect with vertical fractures. Man made include older natural gas wells to deeper formations whose casings may in part be uncemented (as was common in the 50's and 60's) or those whose cement has degraded allowing gas movement. Domestic water wells penetrating methane-containing coal beds will also allow methane to accumulate in well casings and well housings which provide an explosion hazard. A combination of both man-made and natural conditions will also result in methane degassing to the atmosphere.