Cost of the operation. electric, loan repayments (unless its all paid for) rent etc. 4 months ago BTC was worth 2x what it is now, its a big chunk to lose.
I think it's all paid for... Here's a quote from page 2:
"I figure I spent about $1 million building the existing facility and racking and deployment of the mining hardware, and then we probably spent in the range of $2 million to $3 million on the hardware itself," he said. Rent is another $8,000 a month, power $40,000 a month. "I think we spent probably $3 to $5 million dollars total on the operation. It's paid itself off [in bitcoins] many times over already."
Clearly a good investment as it's already throwing off free cash flow. But his fixed expenses are high and will only increase as difficulty rises (assuming he wants to mine the same proportionate amount). He'll need Bitcoin to keep increasing in price in order to continue to stay mining, otherwise they'll be a lot of rigs for sale.