The answer to that question is not straight forward but the way I see without been overly technical. There reasons depend on the individuals behinds those accounts but these are what I suspect.
* *Waiting For The Rise* : I know of some accounts that are powering down but to leave it in their steem account or other kind of savings. I guess they are waiting and hoping to maximize any rise in price.
* *To maintain the lifestyle* : Though this may not apply everyone but it is worth considering especially from minnows and dolphins perspective. Few months ago, crypto investors seem to be living large and dependent of the recurrent income from steem blockchain. But with the price below one tenth of what it was then and rewards not accumulating like before. There may be some needs requiring powering down.
But all these are temporary as a lot of steem from the powering down may end up getting powered up once still some rise which will likely to be soon with hardfork 20 and SMT in the horizon.