Being adult doesn't mean assuming everyone automaticly follows the same social/game table conventions as you. Depending on how they were raised, who they used to game with and what they're favorite color is, I find that its very rare that everyone coming together to play in a new group has the same expectations, much less knows what they are for everyone else. :\
For instance, some gamers expect that you will talk to the other players only in character while others find that obsessive and lengthy in character discussion ranges from pretentious to time wasting to annoying. I don't know whats expected of me in that area because I lack psychic powers.
Even things that you might think of as simple social graces are subjective according to the house that you are in. Do you presoak dishes or put them right in the dishwasher? Or hand wash them right away? Disposal, trash or compost bin? Split the takeout bill even or line item? Answer the phone because you are closest or never answer the phone in another person's house? Ask who's calling before giving the phone to someone else or thats rude? Excuse yourself from the table to use the bathroom or leave without comment? Seat up, seat down or lid down because the cats drink the tidy-bowl otherwise?

If everyone doesn't agree on those, what is the harm of spelling out table rules on how much notice you want before a D&D game night?
Adult /= psychic!

I sometimes think more blameless friction is caused by unspoken understandings than anything else.