Been some pretty good guesses by a few people, I guess Church groups.
Not to offend (I am Christian), but just that was what I have heard before.
One of the “problems” with church groups is that they usually don’t drink and often they don’t even order much, so their bill is low. But all groups tend to be bad - everybody thinks that the tip is somebody else’s responsibility, and a lot of times they are ordering a set menu anyway and don’t have as much personal interaction with the server. That’s why most restaurants put a surcharge for the tip on the bill for a party.
There’s nothing like hustling your posterior off to get 15 meals out at the same time, hot and correct, and then find $2 on the table (from the one generous member of the party who even thought to tip!)