Possibly because the alternative of renting is much easier in a condo? So as sellers realize they aren't getting the price they thought it is easier for condo owners to try to rent, whereas coop owners need to be more negotiable.
For coop owners where many are restricted to renting for only 2 years it is a very big gamble to bet that 2 years from now prices will be higher/the market will be better. And the bet includes carrying the unit vacant at that time either way because their sublet allowance is up. Whereas from condo owners they can just rent again... and again until they like the state of the sales market. Taking a few points less for coop owners may be preferable than taking on increased risk.
As far as why it flipped I think it is simply the realization that prices are headed downwards across the board is more widespread. First it was only in the "super luxury" market. Then in "new construction" (remember that category goes back 5 years). Now I think people realize it's the whole market especially after seeing the Q3 report of how much sub-$1 million contract volume fell YOY.