Please read:
http://fowlfacts.proboards.com/index.cgi?board=afflictiondiseaseff&action=display&thread=1442
There probably is actually 2 things going on.
1) hens instinctively try to protect the integrety of the pack. If another hen has something contagious, the poultry (like all animals & us) try to isolate it rapidly by culling flock themselves.
2) It sounds like a yeast infection that is spreading through flock (see contagious vent gleet).
(birds look/act healthy until you notice soilded vent area).
The more hens are confined in close proximaty to each other, the more rapid the spreading. And the faster hens will peck the rear end out of the other to attempt to contain it.
Probably reason it seemed to stop on own, is once the hens were allowed to spread out ,a) they could get away from infected hens so no longer need to cull it/ b) the infected hens were finally culled out of there.
So maybe consider both:
1) Much More Room ( 12x12 may be fine for roosting, but certainly not for day roaming). Either invest in cage method to keep from coming into contact with environment or free range, but put up with internal/external parasites and preditors. Or at least a large yard as well as 12x12 hen house with 30-50.
2) Give a steady diet as to not upset their digestive tracts which starts or destroys normal bacteria balance, and plenty fresh water. (avoid feeding mix diets or making changes to diets, ie fruits along with feed, or adding anything, ie Apple Cider Vinegar)
Remember, Apple Cider Vinegar can cure alot of ailments.
However if given before an ailment, it can also kill the good & wholesome bacterial balance of the intestines, thus starting a problem later on.
Coomercial producers avoid these problems by keeping all seperated and only keeping no longer than required -3 months for meat birds, 2-3 years for layers.
Whether we like it or not, we are forced to face=the longer in possession, the higher the risk to loosing them to illness, so they eat them first.
I personally am like all backyard keepers and attempt to keep mine much longer.
it comes at steep expense and time costs and more critical it becomes for : roominess, fresh and sanitary the drinking bowls (wash with soap every 2-3 days), The more uniform their diets, (yours and their) habits must be, and the more heartache, time and medicine one will encounter.
I ,like most, fell into not realizing how difficult keeping birds are, say to goats or horses.
But now realizing it takes the same skill as keeping a parrot healthy, multiplied by 30-50 of them.
So give room, don't change daily habits and diets, stay on parasite routine, don't mix ages as this all lends to internal stress that changes gut juice. Like All birds (cockatiels to pheasant) are very sensitive to what seems like minor changes within their general confinement. You often won't notice for weeks-months, then not know what brought it about.
and by the way, make sure to give concrete (patio blocks work well) so they can trim their beaks themselves.
Debeaking is outlawed in Europe for good reason.
Here in states, USDA requires training for it as there is a main vein and nerve root. Debeaking is only done on chicks before nerve root and blood vessel gets to large.
You can start neuralgia easily and many times you have to cauterize, which starts a pain cycle too or they can bleed to death.
Having to resort to debeaking tells others that birds are over-crowed or ill. I have never experienced cannibalism in 15 + years of raising poultry and selling eggs, even when flock sisters were injured. They have plenty of room to stay clear of the injured and trust I'll take care of it. So they don't feel the need to take matters on themselves.
Just follow above stabilization methods and remember, cannibalism is a sign they are attempting to cull someone out before everyone gets ill generally by parasites, but can be disease such as contagious yeast infection, too much light at night making them irritable with sleeping partner, etc. etc.
Good luck to all,
I have to go blow dry my hen with vent gleet from not following own advice and adding stuff to diet like fruit, eggs and apple cider vinegar, garlic.
Never had these problems when I just fed steady layer pellets. UGGGH!!