I posted on this a while back. I don't think anyone believed me then, but this little clip explains things well.
It doesn't mention that brambles will also capture humans.........
Looking at the blogger's land though, he could do with cutting back the brambles from near the fences, prevention being better than cure, and he could also have cleared away the cut bits after he'd released his sheep, so they don't promptly hang onto the next passing sheep and become entangled.
But yes, definitely, brambles are deadly, carnivorous, and out to get YOU