Spotify is designed so that when a podcast publisher needs to update any content of an episode without publishing a new episode, it will update any downloaded episodes (sometimes these episodes are referred to as "available offline" episodes). The number of episodes the listeners of a particular show have made "available offline" (aka a downloaded episode) are unknown until a redirect is placed. So when a podcast is redirected the 'enclosure URL' is updated, which causes all of your listener's available offline episodes to re-download. This is what happens when you see a "spike" in your download statistics. Spotify is aware of this behavior and working on a solution to remedy the misleading duplicative downloads.
Podcast publishers should anticipate a spike in downloads from Spotify variable in size (depending on how many listeners have said show downloaded), when migrating a show and or changing or adding a 3rd party analytics prefix URL.