I had dinner with a friend and suddenly we were discussing how to do the laundry. It made us laugh because we never discuss how to do the laundry. Basically over dinner, we always cover our work (and what doesn't work in work), work/life balance, gossiping over other friends, children and sometimes a bit of politics. Did you ever notice that with certain friends or colleagues you always cover the same topics, though with variations on how you cover it? I once saw the play 'Painicilline' by Alex d'Electrique where 3 scientists are nominated for a price and waiting for the result. It takes long and you see a pattern of conversation between the 3 men that gets repeated and repeated. The same topics come up and get discussed in just a slightly different way.
In communities of practice, the same phenomenon occurs. The same topics are covered and others become 'undiscussables' or simple 'not discussed'. A facilitator can play a role in introducing new subjects of conversation, by changing the medium, inviting new participants or inviting new experts to interact with the community.