Why is my Windows Phone filling up?

I have a 520 as a backup and have been using it for a few weeks with my phones in the shop. It's been interesting.

But I am having a problem. It's filling up all by itself. It was maxed out and couldn't open many apps so I started deleting a few apps and podcasts to free up space. This worked enough to allow the storage check app to finally retrieve the information successfully.

I have 89MB free! Good.. wait... 82MB? 74MB? It started going down. I quickly turned it to airplane mode in hopes to stop the madness but it didn't change anything. Now it's back to 0 free.... just like before I deleted the apps.

Other is 4.75 GB!

It won't let me delete the temp files either, it just stays at one gig.