So, I'm riding yesterday and...

[fourflys]

I'm coming down a big hill, just spinning in my big gear when I lose all resistence on the cranks... I thought my chain snapped! I looked at my cassette and noticed my outer cogs were just spinning... I got home and noticed my lockring came unscrewed. Weird, I thought they were designed to actually tighten (or at least not loosen) as you pedal... Is this common and something I should check on a regular basis?

[false_Aest]

gremlins

[paczki]

The lock ring doesn't tighten as you ride and it loosens from vibration. You need to make sure it's on tight, obviously not so tight you can't get it off.
That happened to me and I started to tighten my lock rings more!

[bronk]

I always do two things to reduce the chances of the lock ring loosening on its own -- (1) make sure the cassette and lock ring are properly seated (ie not cocked) and (2) Use a torque wrench.

[android]

I'm coming down a big hill, just spinning in my big gear when I lose all resistence on the cranks... I thought my chain snapped! I looked at my cassette and noticed my outer cogs were just spinning... I got home and noticed my lockring came unscrewed. Weird, I thought they were designed to actually tighten (or at least not loosen) as you pedal... Is this common and something I should check on a regular basis?

No, the loose for the same reason any bolt loosens. Being worked without enough preload. Torque it to 40Nm like the docs say and hopefully you won't have any more problems. BUT... Sometimes when something loosens on its own, it damages the threads and won't hold anymore even at the correct torque.

[Dekonick]

Never had this problem, but perhaps loctite green and a torque wrench?

[fourflys]

thanks all! :)