This is a great question, and you may have reason to be concerned.
Is your cham doing this when she is sitting under a basking lamp? If so, it is probably her way of releasing excess heat. How are the temps in her cage in general? It could be too hot for her in there.
If she is doing it at random times for no apparent reason, then there may be a problem. This is also an early symptom of an upper respiratory infection. You may want to watch her for signs of slowed eating, lethargy, etc. If you are pretty sure that she isn't doing it because of the temperature, you may want to run her into the vet and get her started on antibiotics.
Either it is too hot in the cage. Most animals will gape their mouth to cool off. When their mouth is open the evaporating salvia will cool the incoming air. This inturn cools off the core of the animal. If you suspect this check the temperature. If you have a basking spot should be 90-110. The rest of the cage should be in the 80. If she is young stay away from the high side of that basking recommendation. They like it hot but the young ones have less surface area to heat up so they can quickly be overcome. If she is real young you might just hold off on using one for a while longer.
Some sort of respitory infection. If the nasal passage is clogged they will open their mouth to breath. You will usually be able to here some sort of wheezing when their mouth is not open. Not really to different from when your nose gets clogged up.
Chameleons will do this when they are about to shed. Opening their mouth wide helps loosen the skin around the mouth. I do not really know with alot of other chameleons but mine like to hiss. They will eat their skin. Veileds have been noted to eat other reptiles skin. Not sure why. Since you said she was just opening it for a minute at a time I would suspect this first.
I guess I would point out something that you may or not know about veileds. Try to keep the basking branch near the bulb but not under it. This can burn their casque. They can not feel it to well and will not know that their head is being burnt. She may still chose to lean under it but they do not usually do that for an extended period of time. My girl still has a little damage to her casque that has never grown back normal.