Not been there for a while, but it could well be the anti mtb squad. Just about every time I went down somebody had laid sticks & rocks on all the mtb paths, usually just over blind drops and stuff. Every time I kicked them out the way, but next time they'd be back.
Very annoying, I think it's some tossing dog-walker or something with a grudge against bikes. I thought about getting up really early to stake it out and give them a piece of rightiousness but in the end couldn't be arsed.