So here's the deal:

Safeties on most of the shotguns out there only block rearward movement of the trigger. That's it. They have no impact on the sear or hammer.

The typical shotgun sees very little in the way of routine maintenance. This is especially true of shotguns owned by police departments. When, for instance, was the last time you remember replacing a sear spring on your 870?

When you combine the weight of a typical repeating shotgun with the unknown state of various important fire control bits, it's entire possible for a significant impact...like from a shotgun falling over...to lead to the hammer coming off the sear and the weapon firing unintentionally.

This has happened a fair few times, especially in law enforcement. Officer Friendly's cruiser shotgun is chambered and he goes over some railroad tracks and BOOM: Unintentional sun roof.

So, generally speaking, we tell people to store the gun with an empty chamber.

The worst that's likely to happen for the average citizen is that the gun falls over when it is leaned up against something. That is less likely to result in an accidental discharge...but given the power of a shotgun, the truly horrific damage they do to flesh and how easy it is to get them chambered and into action it really makes the most sense to store them with an empty chamber in most situations.