Since it's random, make sure it's not just a discharge of potential they build up when walking on the stage. If they can touch the mic and have an even flow of zzzzzap then it seems like it's electrical. But you mentioned that there were people without guitars.
Since I've seen it's story-time, I have one short one. I didn't work this gig myself, but colleagues did, so kind of lame but still. World wide TV-event. TV-Hosts were people you all know who are. Stage floor was plexi-glass. Built up a potential. Solution? Touch the (muted) mic to discharge then speak into it. TBH I don't get this to compute since they were on WL. But from what I've heard there was a mic on stand and this the solution.