It's natural support. When vested steem is greater than approx 90%, then steem power holders have a negative rate of real return. When it's below they have a positive rate of real return.

This is because 90% of the printed steem gets allocated to SP holders and if this is above 90% vested then the return is lower.

Once that level is hit I'd expect buying, just based on structure alone. Or at keast a reduction in selling.