Lost weight distribution bar

I think the retaining system (varies by brand) gets worn out, or possibly just not engaged properly. Then the shank walks itself out the bottom. Add to that the fact that a large percentage of them were never properly adjusted to begin with.

I've seen worn, and even broken, retaining clips but even without them it is difficult to drop a bar out while it is under tension. I guess maybe a large enough bump that would reduce the tension might allow it to drop out? If that was the case it would still be attached by the chain and drag along.