I don't think that most of them are doing that, but there might be quite a few powering down. Regarding the reason on why would someone power down it's hard to tell.
Some of them might leave Steemit for good, for any particular reason, some of them might invest their Steem in other cryptocurrencies while some of them probably are keeping their Steem liquid for a possible future bull run in which they can sell those coins at a good price and take some profits on their investment.
It might also be the case of some people powering down to sell high if prices are going to go up in the next months to come just to buy more coins cheaper after the bubble of prices bursts. It's hard to find an exact reason for every power down you can see on different wallets.
There are could be many strategies in work for every user.