I am sure supply and demand has a lot to do with this situation. But as the OP has indicated, there seems to be now much more frequency of the rate change happening, at least for me.

In the past, when I have shortened/lengthened stays, I don't recollect an rate change.

I have made many reservations of late, and I had initally plugged in dates for the future just to hold a few days or a week, while I summed up arrival and departures. Now that the dates are up and coming I fine-tuned the arrival and or departure as the needs had changed and it seems that with every reservation, the complete rate for the duration of the stay changed.

Dash