Looks like 6.5M shorts covered in the first half of the month. Given that only 12.1 million shares traded for that period one would think there would have been more price action.

Not if those shorts were a collar. That's where you own shares, and want to lock in your price without selling. Basically, you short the same number of shares that you own, so the rise in one position offsets the fall in the other, thus locking in your price. Closing the short is simply a matter of delivering your shares to the lending broker. This happens off the exchange, so it doesn't move the price.

I'd say that's what you're seeing, although I'd only be guessing if I said who was doing it.

And yes, it's legal.

ebear