I know it had to do with methane, but how did an explosion occur underground like that? Only during winter months?
I am a coal Mining Engineer and a Mine Rescue team member.
When areas of a coal mine are no longer used for mining, they are sealed by building very sturdy block walls and a sealant is applied to the blocks. Although this seal is pretty well airtight, some gases can escape through the seal. The strata above and below the coal seam allows methane to bleed out through small cracks that are present in the rock.
When the barometric pressure drops, the seals will almost always "breathe" methane from them. Generally the amount of methane that is released is not enough to reach the explosive limit (5 - 15 volume percent methane). The ventilation that is provided by the main fan sweeps this gas away and does not allow it to accumulate to dangerous levels. However, if a rock fall occurs or some other obstruction is present around the seal area the ventilation path can be blocked thus allowing methane to build up.
They are saying on the news that this explosion occurred at the set of seals where the miners turned on the 2nd Left section. Somehow methane had to have been able to accumulate around the sealed area due to a blockage or some other reason. Something had to set the gas off such as electrical equipment or possibly someone smoking (which is highly illegal).
What seems funny to me is the first person that was found dead was a "mine examiner." These people travel throughout the mine and make sure that conditions are safe to be working in. This man definitely would have had a hand held gas detection device on his body. He probably was not physically in the immediate area where the gas had accumulated but was close enough that he was killed by the devastating explosion.
You ask about winter months...Late fall to early spring is a particularly dangerous time in a coal mine. Cold air that is very low in humidity is blown into the mine. That air warms up in the mine and absorbs the humidity that is in the air, thus "drying out" everything in the mine. This makes it easier for float coal dust suspended in the air to ignite if another explosion occurs, which is probably what contributed to the ferocity of this ignition. Float coal dust acts just like gun powder and greatly adds to the explosive force of a methane ignition.