I wonder if my take on it is right. Why would Doris and Mary and Angie not get along? --- they're not known as difficult people, particularly. Angie and Doris are friends, and Angie has praised Mary's work before.
So who knows? Maybe they're just bored and have run out of small talk.
After taking a second look at the way both the others are looking away and MTM is staring defiantly straight ahead, I've decided that Mary had just farted! :-P