Have you checked your water for amonia ?
Aside of the normal itch a fish can get and as CM mentioned ick can also cause a fish to want to scratch. With ick the fish will do so because the parasite is beneath the skin / scales.
Now if the amonia levels are high this can cause this scratching you see also. If the amonia is high the fish are literaly being burned and will rub on objects in response to the discomfort.
Check the water quality and monitor for any signs of ick.
I would lean towards either or because if it was just a common scratching you would normaly see a fish or two doing so not several. This is normaly a sure sign that something is wrong or about to go wrong. My suggestion would be to check the amonia, and as mentioned monitor for any signs of ick.
Good Luck,
Danny