Who knows really, could also be on purpose (most likely). Only reason I can think of is the person doing the dumping was aware Steem is not on many exchanges and that the dump will likely execute stop losses or panic sells on other exchanges too (like bittrex went down to 5500 sat) which could've made it worth it if they were buying cheaper on the other exchanges combined with most of their buy orders on binance being their own they sold on to. Could've yielded some profit for the person causing the dump in the first place but it's just a theory.