Agreed, the heat, humidity and the amount of wind. Last summer in Kansas bluestem could be cut one morning and baling started the next morning - any longer than that and you were making straw. Also depends on how dry the ground is under the hay. Last summer the ground seemed to draw the moisture out of the hay instead of adding to it.
It also depends on how you cut it. Mowing it flat and letting it dry for a while then raking it into a windrow to dry some more seems to dry faster than dropping it out the back of a swather - even with it crimped.