It appears that there were plenty of shares to borrow and thereby short OPK all day, until a few minutes just before the close. At that point all sell orders to open a short position were canceled by brokers who could no longer find shares to borrow. Those who shorted during the day may have bought back due to the risk of not finding shares to borrow by settlement date. There was likely concern of recalls of borrowed shares as well, adding to the buying by short sellers.
This was a major factor in the 'sell imbalance' that disrupted the close.
For the life of me I can't figure out why anyone would short this stock. There are numerous stocks that are ideal shorts. This by far isn't one of them. I can only think those shorting OPK are delusional. You can't help but feel bad for the crazy folks.