I have experienced this (on MacOS 10.8 and 10.10) since the iTunes with the red icon downloaded (I am currently 12.0.1.26). In fact, I am experiencing it right now.
One scenario that seems to trigger it is that one of my feeds will go haywire and try to download several years worth of old Podcasts. If I stop it by deleting the ones in progress, then it will stop refreshing all of my podcasts. Today an NPR music podcast decided to start downloading four years worth of mini concerts on a hotel Wifi that has inconsistent performance.
The only thing I have found to fix it is to quit iTunes and leave my Mac up and running for several hours (I usually do this overnight). Then the next morning it seems to start refreshing the other feeds again.
I have a few Podcasts that have 4-8 years of episodes and all are still in the feed. It is several Gigabytes if all these download again so I don't want to tie up my bandwidth and I kill it when I see that happening. The real thing that needs to be fixed is why these old episodes show up as new again. I think the changes to Podcasts including having the concept of episodes "in the cloud", or on one device but not the others, is contributing to these glitches.
(Note: I let iTunes on my laptop manage everything. I then put episodes into playlists in the order I want to listen to them and sync the playlists to the other devices from time to time.)