I'm no expert, but I would imagine, for example, x trades, very small volume trades low bids,.Short traders can be  holders while short selling borrowed shares--it is not a given that they have access only to borrowed shares and have to extend borrowings, if the imperative to bear down on the SP to protect a short position exposure  is greater than the perceived benefit of holding on at a stable or even increasing SP they can sell down if only on small volumes.