You're right about herds, seems to flummox a dog at times, also noticed at the start it can fry the brain of a real stock broke dog a bit, not seeing the trees for the wood as it were.
Seems to be time of year though, see roe in 1-3 most of the year, fallow seem to herd up at times, and at others we seem to see lots of 1-3 animal groups. Also I've noticed that once a herd is scattered, they seem to lose their heads a bit and stand about confused in various places.