If your in a raidgroup that has almost the same composition every week, you are most likely gonna know most of the people and know how they are going to react on certain situations. And a lot of times there are going to be jokes/puns.
In my guild and raid group this in no differant, Like we got a rogue that is afk every readycheck, we shout "Cake" on ventrilo and he says: 'yeyeye im ready'. Once we were doing VoA and i was tanking, so all goes fine, at the time emalon was the last one in there, so we charge in. I took emalon the other tank the adds. Nothing wrong you would say, only problem was that i was in dps spec and gear so i took quite a beating. Ever since each time i get a massive beating, they ask me: are you in the right gear gathy?
A good pun is one that you can repeat without it getting anoying/repetitive, of course if you keep on jabbering the whole time about it, you'll make it anoying.
And these things can loosen your raid, it helps helps to relieve some stress after something like say a night of progress, where you just cant down the boss due to small things.