Hi and welcome,
Sorry to hear about your incidents, but unfortunately, it's dogs being dogs. Granted, for the breed, a little out of character, but unless your dogs are properly trained being around livestock, then they shouldn't be allowed really. And by 'properly trained', I mean lead and command trained to ignore them. Just saying it's never happened before doesn't mean it can't happen. There could be a million and 1 reasons why it's just suddenly started to happen, but the fact it's happened implies it's likely to happen again. You could possibly train them out of it, but it would probably be better to keep them properly separated. My old Staffies used to be fine around the chickens, but they were 6+ years old when I first got the birds, so quite chilled and reserved. When we got a new GSD (when I still kept poultry), she went absolutely crazy at the sight of the chickens. I wouldn't have trusted her for once second around them, especially in a heightened state of anxiety. Without knowing much about your dogs or your environment, it's difficult to advise, but know that any training will probably be quite involved and will need constant reinforcement. Dogs tend to get a taste for chickens once they've logged their first kill. Chastising the dog also won't do much to prevent the situation. Punishment is not something dogs really understand, and especially over a long period of time. Within a few minutes, the dog has probably forgotten what it did to be punished, so then becomes more anxious and more unpredictable. A simple telling off, and then taking steps to prevent the issue would be a better option.
Giving power to politicians is like giving whiskey and car keys to a teenage boy - P. J. O'Rourke (thanks Jessie)
It's amazing that people can believe everything is predestined but they still look both ways when crossing the road - Stephen Hawking
1 Wife, 3 children, 1 Staffie Bitch (RIP Marley), 1 Chi-Chi, 1 Tuxedo Cat, 1 part Maine Coon cat, male bearded dragon, Horsefield Tortoise, 2 White Silkies, 1 Frizzle Pekin, 1 CLB, 1 Appenzeller Spitzhauben Cockerel, 1 blue laced Wyandotte, 3 Appenzeller x Wynadotte pullets, 1 Call drake, 3 khaki Campbell ducks, 4 (2 male 2 female?) Aylesbury x Campbells, a breeding colony of Dubia cockroaches.
And a lot of Ibuprofen.