UBSUBS sold about 350,000 shares in the last 2 weeks of August; they are likely selling for the Hedge Fund who they represented over the past 2 years as they exit their final position. If you recall they have been a big seller in the past 6 months. This kind of activity is normal for short-term investors ie: said hedge fund.
Volume should pick up in the next several weeks and a bid should enter the market as likely positive events unfold for the company. (Earnings Call, Ibalizumab Secondary Endpoint & Potential Analyst Upgrades)