At the start of a new Mayor's Pass season, several things happen at once:
- There is usually a game update
- The old season ends, and any temporary storage you had from that season expires. This can be as much as 85 storage slots depending on what you acquired during the old season
- Any unclaimed Mayor's Pass rewards are automatically added to your storage. This can include storage, land, beach, mountain and Vu items
- At around the same time, the previous week's CoM ends, and any rewards you have from the Club chest are also added to your storage - this can be another 15 items depending on your clubs progress that week
All of this can have a dramatic impact on your city storage, and it helps to plan around this by clearing out unwanted items in advance.