Oops...sorry for the double post.
They do this when they have MBD....don't know if there can be other causes or not. I would take it to a vet...they can tell you for sure that it is MBD and treat it.
If it is MBD, you need to look at your husbandry to make sure that the chameleon is getting what it needs. Phosphorous, calcium, vitamin A and vitamin D3 need to be in balance for good bone health.
Is the basking area warm enough to allow proper digestion?
Does your chameleon get exposure to UVB light either from direct sunlight or from UVB tube lights? Neither should pass through glass or plastic. Exposure to UVB allows the chameleon to use the calcium in its diet.
Do you gutload the insects with a nutritious diet? Do any of the things you gutload with have phosphorous, D3 or preformed vitamin A in them?
Do you dust the insects with a phosphorous-free calcium powder at most feedings?
Do you use any vitamins on the insects? Do they contain vitamin A from a preformed source or beta carotene? Beta carotene won't build up in the system but preformed can. There is controversy over a chameleon's ability to convert beta carotene...so some people use one with preformed vitamin A once in a while. Too much preformed vitamin A will work against the vitamin D3 and can lead to MBD.
Do you dust with a D3/calcium powder? I don't think it is necessary if your chameleon gets exposure to sunlight...but because mine don't I dust lightly with D3/cal. twice a month. D3 from supplements can build up in the system...so don't overdo it.