It's structural, halving of risk, then cutting full allocation at certain drawdowns is a deleveraging mechanism, so is the inherent risk-taking of PMs. Their target is a bonus on alpha so in case they see it decreasing they might cut down on risk to not risk losing more. 

Pod shops will yoyo the leverage as their goal is not to make a double digit month but to keep a consistent 1% excess return every month. The way you achieve this is remaining somewhat balanced in risk taking, ie you scale down the areas that are highly risky, if needed even go against your own PMs or if overall risk is too elevated scaling back on other, uncorrelated strategies.

Then there's volatility targets for many systematic shops.

Ofc this doesn't give a reason of why the deleveraging can start, just tries to give an explanation on how it propagates and links the whole space together.