I think I know where you’re talking about. I don’t know if the landowners are behind the signs, but they have been annoyed at some of the cheeky in the past year and that’s been discussed on some local MTB Facebook groups. Mainly it seemed to be the blind exit rut track spilling a pile of dirt into the road that tipped them over the edge.
It might be someone else though. Lockdown seems to have really brought out the wannabe trail cops everywhere, it’s like New Year on steroids: more people out more of the time, some now all indignant about things they otherwise wouldn’t see for most of the year. There’ve been people putting up unofficial NO BIKES signs on Yorkshire Water land