Normally within any "angry" group there are always one or two "ring leaders" and the rest tail along behind.
So you need to look at the team and see which of them may be the more likely to organize this sort of thing.
It will probably be the most "vocal", those that moan the most, or those that may have been trade union leaders or similar in the past.
Then have quiet one to one meetings with the most likely suspects.
DONT let them know you suspect any secret meetings, just have a general chat about how happy they are, any problems, also ask them how you are doing etc. This gives them a chance to air any problems.
Also find the most TIMID people in the group who may be being dragged along by the strong ones.
Now have a one to one meeting with them, try to get them in your confidences and ask if everything is alright on the "shop floor" and so on.
That way you attack the problem from both ends.
I would then have a chat with the ones "left out" so nobody suspects anything.
That way you have given them all the chance to get things off their chest.
To be honest regular informal meetings with each of your staff should be an ongoing process to try to nip things like this in the bud.
Try to have open and honest one to one meetings with your staff every 3 months, even put the dates in the diary to make it more formal.
Hopefully they will then moan at you rather than moaning at each other and you can try to sort out their problems.