Disgusting, but becoming more common. I've seen similar on Bodmin moor and parts of Dartmoor. Possible culprits are unscrupulous builders or 'man and van' type operations where they otherwise have to pay to dispose of 'commercial' waste at landfill sites. Quick trip to a remote spot at night, get rid of the rubbish, no expense, job done.
Doesn't help tho' when council sites (eg weston mill in Plymouth) have signs saying 'no vans' at their entrance - I turn up with a Landrover full of old lath and plaster, get turned away and then have to transfer it to my car to dispose of. I suspect tho' that the 'private household waste' facility has been abused by the commercial sector. Will cost the council more to go and collect it from the Moors tho'.
Nick