What is driving his consistent sales out of banks, I wonder?

Who knows?

Warren could have sold a lot more Bank Of America in Q4 than he did, so it is possible his primary goal was to get the position below 10% so he didn't have to file Form 4s publicly. He went far enough below 10% (Berkshire owned 8.865% of BAC at year-end) that he doesn't have to worry about the effects of BAC share repurchases for a while.

Berkshire has approval to own much more of BAC (I think the 2019 approval was for up to 24.9% with certain limits on their commercial relationship and influence) but that doesn't mean he wants to be stuck filing Form 4s within 3 days if he wants to sell shares.

It is possible that the Citi investment was not Warren's. It very well may have been Warren's position but it could also have been Todd Combs' position. I would be very surprised if it were Ted Weschler's position for several reasons. The Citi investment may have just been straight valuation - it was way too cheap, got cheaper, then rose closer to fair value and they started selling. If it was Warren's position I bet it will be completely gone in Q1.

Capital One is Todd Combs. (not disclosed as such but I would bet on it)