Either they got "hacked" or they misappropriated the funds. Bitfinex got "hacked" back in the Summer of 2016. I highly suspect that they have misappropriated funds, though, and the "hack" was a inside job. Even if it was not an inside job, it takes a special kind of stupid to allow some hacker to access the hundreds of private keys that was required for the "hack." It may be really bad for the BTC economy when they finally go under. However, in the end, the cryptocurrency marketplace will be better without them. I absolutely dispise Bitfinex.