All I can say is, going into this with 30% cash in the PP is AWESOME. The Bogleheads portfolio in my retirement accounts, not so much :-)
All these posthoc, know-it-all explanations of "why" the market is dropping are entertaining, but useless. They can't really know, not without conducting a large survey. This event could have started simply as a localized dip in an early-opening Asian market, then people already a bit rattled by something (tax reform, interest rates, government shutdown, people just afraid the stock market went up too fast and my buddy is going to sell so I'd better do it first, whatever) see it and start following suit, then more people take notice and make "me-too" sales, and then automatic sell signals kick in, and there you have it.
One of the things I study is biological networks, and it occurs to me that it's a good explanation of stock market behavior. People tend to think of networks as organized from the top down, like an orchestra: there's a conductor, the violins are listening to the cellos, etc. A better analogy is how geese come to fly in a V formation. They don't decide as a group to form the V. Instead, each of them has its own localized rules for where it wants to fly in relation to the goose next to it. I call it a self-organizing network, and it's incredible how much complexity can arise from a few very simple rules. You can try to make up rules for why the large-scale structure forms as it does, but you'll fail because there aren't any. There are very, very few people who have recognized and truly understood this, AND were willing to admit it.
Given this, sticking to a formula that is "good enough" and works under most conditions is about the best you can do. I think the PP is working just fine in this situation by limiting losses, and putting you in a great position to take advantage of the stock sale. People who are 80% stocks can't do this, because they don't have the necessary reserves. All they can do is wait for the market to go back up. Could be next week, next year, or next decade, no way to know.
Thought for the day!