Apple Podcasts may download old episodes due to several reasons:

1. **Subscription Settings**: If your settings are set to download all unplayed episodes, then the app will automatically download old episodes that you haven't listened to. You can change this by going to Library > Shows > [Podcast Name] > Settings > Custom Settings > Download Episodes, and select 'Off' or 'Only New'.

2. **Unfinished Episodes**: If you have started an episode but not finished it, Apple Podcasts might download it again so you can complete it.

3. **Re-subscription**: If you unsubscribe and then re-subscribe to a podcast, the app might download old episodes.

4. **App Glitches**: Sometimes, the app might have a glitch that causes it to download old episodes. In this case, you can try updating the app, restarting your device, or reinstalling the app.

Remember, it's always a good idea to keep an eye on your storage if you're downloading podcasts, especially if you're using a device with limited storage.