Submitted as a bug to Apple:
When a podcast is not listened to for either five new episodes or five days, iTunes will take it upon itself to stop downloading the podcast, even though the user has not directed it to behave in such a manner. Neither is there any user-accessible setting to turn off or adjust this behaviour when it is unwanted by the user. This is an improper behavior for iTunes to take, considering users may wish listen to their podcasts in batches at a frequency longer than Apple’s fixed number of days and/or episodes.
Thankfully, an AppleScript has been created by Doug to work around this by batch-updating all your podcasts. Also useful is the script that allows you to batch download missing episodes for podcasts that may have been skipped.