I seem to remember someone telling me that farm labourers were (not sure if they were conscripted) asked to go to Wales to replace miners that were conscripted to France for tunnel construction. Could this be true ? If so it would explain why so many went from the same village.
There was no such thing as conscription to the mines in WW1. However, the number of miners volunteering in the early years did cause some shortages and later on, when conscription came in, men who had been granted conditional exemption from military service were often required to carry out work considered nationally important instead, including mining.
However, you have to bear in mind that once conscription came in every man between 18 & 41 was deemed to have enlisted for general service or in the reserve, unless they were exempted. If they were already coal miners at that time then they remained exempt (although this only applied to certain classes of miners), but as the war progressed the government asked mine owners to release more men.
The Miners Federation ultimately agreed that all 'Class A' men who entered mining after 4 Aug 1914 would be released first, after which it would be extended to all mine workers, based on certain criteria.
The point I'm trying to make is that men could not simply 'choose' to work in the mines after 1916, so if they were replacing conscripted men then they will probably have been instructed to do so by their local tribunal board as a condition of their own exemption, hence my suggestion that you check for tribunal records.
If they went after 4 Aug 1914 but before 1916 and were medically fit (Class A) then they will have been released to the military per the Miners Federation agreement, unless they then applied for & were granted a special exemption.