Carol is right... There is a lot of debate as to what exactly causes it. I have had it pop up once or twice in a few of my females. Based on my experience and how I feed and supplement I went off of all supplements and just used plain calcium for 5-7 days and it all cleared up.
There are lots of reasons for gular edema. Over supplementation from synthetic vitamins is probably one of the most widely held theories. Some say it is from synthetic Vitamin A, others say D3, or a combination of both.
There are also causes from organ damage/failure. Heart failure will give you edema as will liver problems.
Sometimes gravid females will develop edema.
Some species are more prone to edemas than others, particularly the montane species.
I've dealt with edema in some wild-caught quads. Some of the quads have had edema and others have not. Same food, same supplement schedule.
My vet suspects one has edema from heart failure caused by lung worms. We don't know for sure she has lung worms, but I did witness two choking incidents with that particular baby chameleon, so respiratory failure caused by the damage of lung worms is in the back of our minds. (This is a wild caught from the same shipment as the quad I lost to lung worms. She was housed with the lungworm baby that died.) She continues to grow and thrive despite everything. She came to me at 12g just two months ago and now weighs 38g. She is an eating machine with a superb immune system. Her edema is finally resolving. I'll be having a good look at her lungs whenever she dies, hopefully from old age many years from now.
I have found that when I over dust with plain calcium, I can cause a flare up with a chameleon that has been dealing with edema. I also found that one particular order of crickets seemed to cause an immediate flare up, even when I didn't feed them to the chameleons for many days after purchase. I try not to feed new crickets for at least a week after they arrive and have been fed good, healthy food so that their gut is clear of any synthetic vitamins the supplier fed them. Since I switched cricket suppliers, all the edemas have been resolving. I suspect that the synthetic vitamins can be stored in the bodies of crickets, long after they were fed it.
I am fairly new to chameleons, so my dusting/supplementing was a little heavy handed. I suspect if I had supplemented much less, my edema problem would have been a lot less, although it never was very bad.
My two veiled chameleons have never had edema, no matter how much I have over supplemented or which crickets I fed them.