Hormones?
It might be that the younger one is unwell and the others are picking on her?
I had an older boar whose temper changed, the vet thought he had had a stroke... There are any numbers of reasons unfortunately.
It might be worth separating them all, giving each of them a good check over for signs of illness (keep the castrate with the younger gilt as he's the least likely to want to fight) Once you're sure both groups are healthy and happy re-introduce them.
When putting them back together I'd do it at evening feed, spray them all or give them a wipe with something strong smelling (TCP, tea tree oil etc) and give them a drink of beer (this is the smallholders equivalent of jabbing them with stresnil - something commercial guys do if mixing groups of pigs) Keep a close eye on them and see how they go. I was always told that biting at ears/neck isn't as serious as biting at the back ends (probably due to potential injury and recovery times) Give all the cuts a good wash and either spray with antibiotic spray or put some sudocreme on them - watch for infection.
HTH