^^Various & sundry. It is where the term 'founder' originally came from, as in a foundering ship = sinking. Founder is more commonly thought of as 'rotation' these days, but can be either or both.
Can be metabolic - why fat ponies are most commonly associated with the risk, can be purely mechanical, can be from stress even. Just attended what I'd call a catastrophic founder today actually - It's as if the walls have 'melted' through, in a sort of step(no good at explaining but sure people have seen similar pics) and the soles were bulging, horse very lame... turns out she is in foal & was AI-ed, suddenly suffered acute laminitis after that & ever since. I don't know if it's the drugs, hormones, stress or combination, but it's apparently not uncommon for mares to suffer this after AI!