From Reuters:
Investors can expect a surge in volume at the close of trading on Friday, when Russell Investments announces the annual rebalance of its series of indexes that will affect more than $5 trillion in assets…..Credit Suisse anticipates about $42 billion will trade on Friday as a result of the reconstitution, which will make it one of the biggest trading days of the year in terms of dollar volume.
Read more at Russell shake-up likely to drive heavy volume at Friday's close | Reuters.
Ah, that explains the spike. Thanks!