Sounds like a "market failure" waiting to happen. So everyone collectively sees the ratio is under 90% so they powerup, which they bring the ratio over 90% so they collectively start powering down, but it takes a long time to power down so STEEM POWER holders are losing out.
Does STEEM DOLLARS count as holding STEEM? If it does, that is an interesting dynamic. If the value of STEEM drops, that means STEEM DOLLARS held will increase nonPOWER STEEM held by more than 10%. POWER STEEM becomes more attractive, causing people to want to power up.
RE: The Push and Pull Effect of Steem Power Incentives - Why the % of Steem held in Steem Power will tend to 90%