When you are on a dose of steroids for longer than two weeks your adrenal glands go to sleep and a big portion of the tapering process is to get them to take back over the natural steroid production. Read up some on adrenal insufficiency to better understand. You can even to a Mayo Clinic Connect search right here in the PMR chat room to read what others have said. Go to the second Search box on this link to limit search to PMR Chat Room. You can also Search Tapering or Titrating - a lot has been written to the topic.
https://connect.mayoclinic.org/group/polymyalgia-rheumatica-pmr/
Also, keep in mind that the lower the dose of prednisone you are on the greater the percent of decrees 1 mg can be. At 20mg/day a 1mg reduction is 5% reduction at 10mg/day a 1mg reduction is 10%. Your body may be reacting to what is sees as a drastic reduction. Separating the adrenal insufficiency symptoms from the PMR symptoms is next to impossible.