Ann: Successful launch of US Trading, page-15

So either:

1. A prospective accumulator, wanting to short to manipulate it down, to accumulate cheaper. Price rises in the medium term, and you gain the extra fee too.
2. A regular shorter, expecting it to fall. In which case, you'd be losing anyway - being wrong by being long, but lending the shares offsets a bit of your loss.

The shorting only brings forward some sell demand.
Your shares get sold today, driving down the SP today.
Your shares have to be bought back at a later date, adding some buying demand at that point, driving up the SP.