Interesting!
I'll take a guess without googling.
My guess is based on your examples is that it means bringing up issues that are embarrassing, controversial or otherwise apt to break social harmony that have already been resolved a long time ago and where an informal agreement exists to let them be.
In short, when you say "stop taking old cows from the ditch" to someone you're chastising them for not letting bygones be bygones.
Hey! Thanks so much for responding :D As promised, here's the explanation!
Your guess is veeery close, haha! It can literally be any type of story that has happened in the past. Can be something embarrassing too! The situation is almost forgotten, but not quite.
Apparently it's origin is that farmers had to fish out the dead, drowned cows from the ditches.