I thought it was pretty simple - they fall asleep. Tim Stockdale showed me one of his that had it. It was dangerous when they were picking up a foot because it could fall asleep in an instant and drop on them. But he showjumped it quite successfully. As long as it was kept active they could keep it awake! It was a grey, I can't remember its name.
That mare was amazing! I loved her so much!!! - Cloudy something was her show name.. we just called her the chloeasaurusrex! she was a giant! she was hilarious... u'd hear a thud all of a sudden and there was cloudy gettin back up looking a bit embarrassed!!!!
Boris developed it last summer, but I think it was brought on by the pergolide and heat. It was only a real problem when being tied up, although every so often he would look very suspect in the field.
I have seen it as a result of chronic pain in a number of horses. Usually those horses dont lie down very often so dont get enough REM sleep to maintain themselves. The horses I have seen it in were mostly older horses suffering from arthritis - and often it stopped once they were put on bute. I did however see it on a 4yo mare who was (according to the owner) completely sound. when I was called she had been doing it for over an hr and appeared like a horse under heavy sedation - missing heartbeats irregularly. 4 days after I came out to see her in this state her owner brought her in for a lameness exam - diagnosed stifle DJD. Medicated stifles and at follow up one wk later owner told me shed just realised that before the medication she had never seen the horse roll or lie down.
So, while it can be idiopathic, more often or not theres a real reason behind it.