For several reasons:
Number 1, Ned's steem is only worth about a million dollars, which is tiny compared to the billions that move in the crypto markets every single day. Literally tiny.
Secondly, steem has been moving pretty much with the rest of the market. It has been around 40th place on coinmarketcap and it remains there now.
If steem was falling due to a whale dumping it, you would see steem dropping on the coinmarketcap list as it would fall harder in relation to everything else.