Powering down means changing "Steem Power"into "Steem.
So, if a whale is powering down, this means there will be a huge amount of liquid Steem back on the market. Liquid steem can be easily converted back into other cryptocurrencies or fiat.
When a 'regular' user is powering down, it often means that he's cashing out the profits he made on SteemIt.
RE: Huge Powering Down Proves SteemIt Lacks Transparency. Can We Still Trust SteemIt?