It's because there is a flaw in the Bancor model and it's allowing the whales to game the system & take profits. Whales are buying at the exchange and selling to the contract small bit by bit due to the price discrepancy.

This drop will continue till the 120k ETH floor is completely gone. Then nobody knows what will happen. BNT can go into a free fall and never recover or it can start rising again. Nobody knows for sure because nobody can predict the market.