Why am I getting reminders on my MacBook Air that are not mine?

Are you seeing a badge with a number on your Reminders app or a Notification with a Calendar icon?

Any Scheduled Reminders when you open the Reminders app?

Are you using a Outlook, Hotmail, Exchange, or Gmail email account that is syncing Calendar events?

Add or delete calendars on Mac - Apple Support

Add, modify, or delete events in Calendar on Mac - Apple Support

View if anyone else is logged into your Apple ID at  > System Settings > [name on top], and scroll to the bottom to see if there are any other devices listed there.