Breeding Beardies

[Pirarucu]

It's one thing to say that they do well in groups in captivity, it's another to say that they are communal in the wild. I have yet to see any pictures or videos (or firsthand observations) of Bearded Dragons living together in the wild. I have only seen a handful of pictures showing more than one of them in the wild, and in most of them they were engaged in territorial displays. (And the movie Holes does not count as evidence of a communal lifestyle either.)
I'm not saying that they can't do well in groups, nor that they may not decline and go off food when they are separated from their group. What I am saying is that this is not necessarily evidence that they are communal. Most stores selling young dragons keep them in cages with quite a few others, correct? So all the dragons sold are simply used to having a lot of other dragons around, and when the others disappear the assumption must be that something happened to them and the area is no longer safe. Alternatively, as Tsubaki suggested, seeing others eating may encourage a dragon to make sure it gets a share of the food.

A friend of mine on another forum bought a group of four baby Nile Monitors a while back, with intentions to start a breeding group. After a couple months he decided against it and sold all but one. Same thing happened. With the group they were active and inquisitive, but when the group disappeared, the remaining individual dug a burrow and refused to come out if people were around.
Using your logic, this must mean that Nile Monitors are communal in the wild, yes? I bet they even hunt in packs...

The point is, you cannot use captive behavior to extrapolate their wild behavior or social structures. A lizard in a box acts like a lizard in a box, not a lizard in the wild. It's more likely that this "communal" behavior you are observing is the result of the way the animal is originally housed. Nurture, not nature. I'd be willing to bet that if you housed a baby on its own from the start, you would not see any longing to be part of a group, and if it were introduced to one as an adult it would most likely either shut down the way you described or become hostile to the others.