A couple things can lead to popping a circlip:
-the chain line is off and pulling the cog to one side (probably not if you've been riding it without problems). Check the drive train for wear though.
-the circlip was not fully seated (possible, especially if you were trying to max-out spacing using the spacer rings.
-the circlip is stretched (possible, especially if it has been on and off the driver for repair a lot).
-something damaged the lip around the slot where the circlip sits on the driver (very unlikely if you haven't had the wheel off lately).