Because probably someone plugged it on your switch.
Simply speaking, the AP queries the MAC of the AP identified as rogue and then it checks whether the MAC of the device is physically connected to your network or not.
I suggest you first, identify the target switch port by scanning the wired LAN to find a device with the rogue's MAC address.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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