This is unfortunately not an uncommon thing to find on the trails.
Last year at Franklin Falls (Sniper trail), someone had strung up webbing across the trail. It was chest height and just following a corner. I'd never ridden the trail so was looking pretty far ahead, if I hadn't been it would have been a real nasty surprise.
There is no excuse for this sort of behavior. Multi-use trails are meant to be shared. While we may not share a particular user groups passion for their sport it's obvious that we shouldn't try to make their experience worse. This brings to memory the numerous incidences of trail sabotage that have been going on in west coast communities for years. We've been fortunately spared from most of this behavior.
In light of this happening just an hour or so away, please remember to be careful out there. Keep your eyes up, just because you have a trail memorized doesn't mean it's not different this time.