Are they adult roosters? Those girls are way too young to be with grown up roosters. The roosters will try to breed them by grabbing the back of their necks and mounting. He uses his spurs and his grip on the neck to hold himself in position. It is not gentle, the roosters can kill those little...
The key question is, how old are your roosters, and how old are your girls. Adult roosters will try to breed immature pullets if there are no adult hens for them. This can cause serious injury to the pullets.