So sorry to hear this and hope this type of thing isn't a "new craze"...but... here in VA we have had a big problem with sabotage of that sort and worse on our Mt bike trails....): (The worse being bridge supports cut 90% through...luckily they have all been found before someone rode over the ravine...)

http://www.more-mtb.org/forum/showthread.php?t=25005

so sad.........