It does seem to happen like this in the days leadi

It does seem to happen like this in the days leading up to a Nader deadline. The expectation was that Nader would get the BLA done by the end of March, I think some feel it is safer to sell before with the expectation that it will not be done and buy back after the price drop. There is just not much confidence in Nader's timelines.

The other reason might be that the market is jumping for a couple of days and some investment might be pulled to take advantage of other opportunities.

The third guess is that many times large investors will "test" price action on a stock to see how it reacts to volume changes in buying and selling. This is to asses how much of a short position or how large a position should be held for insurance reasons. Not all shorts are just shorting, most are using short positions to protect short term a larger long position.