This seems to be the key bit:
The rise in popularity of “North Shore” style riding in recent years has inspired some riders to modify the landscape in ever more dramatic ways. The discipline, which originates from the north shores of Canada, involves incorporating elaborate man-made obstacles into the trails – a trend which the MoD says is a step too far:
“We have found that a number of biking ‘courses’ have been created on the estate without our knowledge or approval,” continued the MoD spokesman. “This not only increases our risk under our duty of care, but will almost always be an offence under the habitat regulations, which protect the SSSI and SPA status of the training areas.