Possibilities: short covering, someone taking a big position, or both.

Maybe as stated before some entity has managed to snoop out some clinical trial results, or, a deal is in final states of negotiation and someone working for the other company has leaked the news, or in some other way somebody has found out something positive.

Or maybe our pet shorts have realized there is no good outcome for them if they stay short over the weekend for possible huge news next week, and they're going to move on to dump or pump some other company.