Google Calendar by default adds reminder via email and popup to a new event. The default email and Pop-up reminder time is set to 10 minutes.
If you do not want to receive reminders for an event then you can delete the reminders while creating an event or by editing an event. And if you want to turn off the reminder for all the Calendar events then you can do the following.
Click the Options icon and select Calendar settings from the menu list.
In the Calendar settings page, navigate to Calendar tab and click the link available under Notification section.
In the Notification page navigate to Event Reminders section and remove the Email and Pop-up reminder.
Click the Save button to confirm and save the changes. This would disable reminders for your Calendar events.