My dog is a small mix breed. She was raised along side my chickens since I got her home as tiny puppy.
For 2 years she was great with the chickens and the chickens even would perch on her back to go to sleep in the sun! I used to leave them all together during the day when I was at work - all free range in the garden. The dog protected them from cats and other animals.
But one day I cam home to find she had killed 2 chickens and was chasing the others around the garden. She was like a different dog. I have no ideas what made her 'snap'.
Since that day she was always wanting to get the chickens. I had to lock the chickens in a run - but the dog would pace up and down and try to rip out the wire to get in to them.
I thought I would have to re home the dog - which I was very upset about.
But I put in lots of training - throwing water on her every time she showed interest in the chickens or got to close to them. It was slow going. One day a hen got out of the run and i was right there to see my dog grab it - I freaked out and grabbed the dog by the scruff and shouted at her and shook her onto her back. My dog is very sensitive and I have never done anything like that before. After that time she has never bothered the chickens again!
This was 3 years ago and now dog and chickens are all happy together again - just like the old days. But I always lock the chickens back into their run if I have to go out - just to be safe.
I like to advise people that even the most well behaved dog can suddenly change if something triggers their instincts - so be careful.
Most likely a squabble broke out and it clicked in her hard wiring of what a dog is supposed to do in that situation (natural instinct for most, be it for prey or even other dogs, is to attack what's fighting). Even the best trained therapy and work dogs can get this response tripped, and it's incredibly hard to unlearn something like that once they've gone in that direction.
While I know you didn't mean to go the route of rolling (aka putting her on her back) and it's an understandable response in that situation, I want to point out to anyone else on here that rolling's a bad idea. It can make the situation much worse and can cause underlying (or obvious) fear and even aggression in the long run to either the situation/object, or the handler.
It's good to hear she chilled back out We have to manage our three with our chickens as well - if the dogs are out, the chicks are in their tractor. If the chickens are out, the dogs are inside.