Figuring out why conspecific aggression happens is a really tough one. It's the second reason why bearded dragons are brought into surgery at the hospital where I used to work, right after impactions. I've seen tens of thousands of mutilations and many of them occurred between very well-fed individuals.
I've seen lots of beardies having impaction surgery to remove a cagemate's bones from its gut and there was a lot of food in there at the very same time. It's my opinion that how much you feed them isn't that big a deal. I've seen more well-fed lizards become aggressive than obviously underfed ones.
That might just be because a well-fed lizard has the strength to do more serious damage, though. It could also be that the folks who seek surgical care of their animals are the kinds who feed generously. If you don't have the money to feed your lizards well, you'll never have the money to go to a vet hospital to get a surgical repair of the bite wounds, after all.
The most common thing I've seen in the pre-op questionnaires was a change in the environment. My guess (and it's only a guess!!) is that changes in the social situation cause beardies to have to reassess their pecking order.
Here are some of the things that pop up frequently on those questionnaires:
New cages or cagemates
The owner just bought the animals and set them up in a new home thinking they'd get along fine because the breeder/pet store didn't have any aggression problems
A new clutch hatching
New cages set up where the others can see/smell the new arrivals
More than 5 hatchlings kept together
One sibling from clutchmates housed together outgrows the others by a fair percentage and turns bully
This is no scientific study by any means, though. We always have to go by what owners tell us their husbandry is so there's lots of missing information and no scientific controls. Take it with a large grain of salt.