As someone mentioned in another post, they need a lot of working capital to cover daily cash out. If a merchant gets paid in bitcoin and wants payment in $, Bitpay doesn't go sell that bitcoin immediately and then pay the merchant, they pay the merchant then have to go sell the bitcoin. That mean working capital to cover the outflow.

Additionally, they have been growing and spending like crazy. They are at every conference, opening up international offices, hiring dozens of people,  buying ads, etc. All that takes additional funding.