Powering down is like an indirect way of withdrawing your steempower or let me just say is a way of withdrawing steem,if more people are withdrawing steempower it means they are selling their steempower to get steem,and when they are powering up then it means they are buying steempower with steem
Well going by that analysis then it means that powering down can affect the price of steem negatively because it will mean that people are selling their steempower which means there might be less demand for steempower or they feel their steempower is not worth enough money thereby making supply to be more and demand should be less thereby making the steem price to go down,i might be totally wrong but that is my own opinion concerning the issue