A Large part of this short interest is probably holders of the convertible debt with a conversion price of $8.75, essentially selling their 'to be converted' stock, before actually converting it, via a short sale.
They really would have no risk from the short sale, as if the price keeps rising, they simply turn in their debt and hand the stock from the converts to cover their short sale.
If they sell short at $14, they have in essence booked a profit of $5.25 vs their $8.75 convert price.
With about $400 Million of such debt, there is over 45 million of such shares. This is probably not most of the short interest, but I guess a significant portion is.