Just like with the offering last July when they pushed the stock from 4.40 offering price to 8 within a few months, I think they (BCRX) are making friends with the institutional holders and getting them cheap shares to easily make profit on.
Case in point: the stock reaction to the very positive HAE news was held back due to the looming offering that very same day. They loaded up loyal institutions and holders with the offering at $10, giving them ample room to make a nice return in a short (or long) period of time. From $10, they already have a 17% return here. The Baker Brothers may have been one of those buyers, with 5M shares being added. But who knows if they have been buying in the open market as well to get the additional shares.
It wasn't a mistake. They had that offering ready to go at the close the very same day the HAE data was unveiled to the public.
And honestly, I'm ok with this. It's a good strategy to get in the good graces of the big money players of the market. You make them money, they will make you money. And so far, we're reaping the benefits of that.