As to the question as to whether the late Friday selloff was "predictable" or not, I would say that strong moves on Friday afternoon are more the rule rather than the exception.
Why? I have two theories. First, big money wants to be positioned for the weekend, and since there is no trading until Sunday, late Friday is the only chance left. The final hour tends to be direction already, but on Friday, the effect tends to be stronger.
The second might sound a bit conspiracy theory minded (and empiricists like me revile conspiracy theories), but my years of market study have taught me that Richard Wyckoff's admonition--pre-SEC--to see the market in terms of one mind at work makes some sense today . Late Friday afternoon upswings help spur Monday morning buying, which then gets sold; and late Friday afternoon swoons help create fear and loathing over the weekend (on ET, "Black Monday" threads appear, and not all are sarcastic). Traders (NOT investors--most investors don't give two hoots about the market--I am speaking of traders who know just enough to be dangerous with their accounts) then panic and sell on Monday morning. They sell to the strong hands, who get some decent bargains in the process.
Friday's action reminded me of that of July 27. I remember that day well. The market made an attempt to rally, and then around 3:00, the bottom dropped out, and the market just fell like a rock. I had had a crummy day until that point, turning a plus 6 lunch into a -8 afternoon, but made back my losses with a 20 point final hour (a comeback that Michigan and Ohio State could have used today!).
Monday July 30 opened down, and then rallied strongly.
I'd expect the same this Monday. Early weakness is to be BOUGHT, not sold, imo. Good luck to all.