It’s most likely a walker/hiker that don’t like the trails being dual-use and assume the only group that should be allowed to use them is, of course, theirs.
The sabotage looks to be annoying more than dangerous and mountain biking involves the risk of new obstacles being introduced onto the trails without warning so I can’t imagine I’d be doing much more than clearing them at this stage. Everyone should learn to ride in a manner that allows them time to stop if they see something they can’t get past.
If I saw someone placing them in the trail, I’d snap a pic of them to pass on to the authorities and wouldn’t think too much more about it.