If you added files on another device and have file syncing set to download files "as needed" in the Sync settings, Zotero will need to download them when you open them, and you obviously need to be connected to the network for that. If you set it to "at sync time", it will download all files in advance.
Upgrade Storage
If you added files on another device and have file syncing set to download files "as needed" in the Sync settings, Zotero will need to download them when you open them, and you obviously need to be connected to the network for that. If you set it to "at sync time", it will download all files in advance.