https://news.bitcoin.com/bitfinex-bitcoin-cash-deposits-with...
Bitfinex (unwisely) specified that during the Bcash-Bitcoin fork, lenders would get Bcash for lent-out Bitcoin. However if Alice lends 1 Bitcoin to Bob before the fork and Bob sells it to Carol then Bitfinex will owe both Alice and Carol one Bcash after the fork, but they will only have one to distribute. So they split it up.
This can get worse when Carol lends it out again, etc, etc, and in the end Zack withdraws the Bitcoin. Unbounded liability and potentially no assets. So them paying out 15% less than expected was a pretty good result.