I guess I have a simple explanation

If you look a little bit back into history, you will easily see that the Bitfinex cold wallet balance has gone down to pretty much where it had been earlier this month. To me, it just means that many people first moved their coins to Bitfinex in the hope of receiving SegWit2x hard-fork coins (with the purpose of selling them immediately before they turn to ashes at the dawn). Now that the hard fork has been canceled, it may not make a lot of sense for all these people to keep their funds at Bitfinex any longer. So they obviously chose to withdraw the coins. The Tether controversy may have been the last straw, though