I don't know about the mines in those times, but in more modern times some of the mines I've been around had "change houses" (which sometimes also had showers) for the miners to get into and out of their underground work clothes, boots, hard hats, etc.

When I first heard this song about 25 years ago, I assumed it meant the strikers were getting rid of all the blacklegs' tools and the clothing they wore while using those tools - effecting their ability to work.

I suppose a more severe interpretation, as mentioned above, could be that they stripped them naked as a warning ?!?