Google-calendar – Cancel events in bulk in Google Calendar

Follow these instructions to remove all reminders from all events: Disable notifications for all events on a calendar.

Edit: The following part from that page may be helpful:

To turn off all notifications for Google Calendar, you can change your default settings for your calendar. Since notifications sync between your mobile device and your computer, you can change your settings from either the app or a computer.

Mobile directions

• Open the Google Calendar app .
• Go to the main menu in the top left .
• Choose Settings at the bottom.
• Choose one of your calendars, like Events.
• Touch a notification and choose No notification. Repeat for each notification.
• If you have multiple accounts or calendars, repeat these steps for each calendar.

Computer directions

• Open Google Calendar.
• Click the settings gear in the top right .
• Select Settings.
• Click the Calendars tab at the top.
• Click Edit notifications next to your calendar's name.
• Click remove next to each notification.
• If you have multiple calendars, repeat these steps for each calendar.

It's a time zone conflict between your Google Calendar and the devices you sync it with. I had the same problem. One of my devices didn't have Daylight Savings set, and after syncing it would send a different time back to the Google servers.