The predators clearly had superior strength, speed, endurance and agility. The predator could lift up Arnold with one arm and jump from tree to tree etc.  
I would surmise that they have the strength to do this BUT I agree; being able to rip out the spine "cleanly" does seem unlikely.  And it probably would come out in pieces (not as a whole unit)
Maybe they break all the ribs first..?  
And no, I've never seen an explanation of how they do it.