A little bit of searching and I found this…Spotify Premium reserves the right to insert ads on exclusive podcasts, and ones that they produce/own. Ads will never be inserted into music streaming.

My best guess would be that since they allow ads for podcasts, they are throwing in Spotify pushed ads on podcasts they own. Do you happen to know if it's specific podcasts? I would probably unsubcribe if I was randomly getting ads. The only other thing I found was people still getting ads when they were using air play, but that was a desync bug.