I'm pretty sure its a line tension issue. I did some experimenting and I'm actually able to cause it to happen by thumbing the spool as the sinker hits the water.
The line tension pulling up on the rig/clip is enough to override the water pressure on the clip paddle. The rig stays clipped.
I found if you wait until the rig is about to hit the water and suddenly drop the rod down while stopping the spool there is enough slack line generated to allow it to unclip.
I wouldn't have though it would happen on fixed spool reels, but I guess if there is enough side wind howling down the beach, there might be enough tension on the outgoing line to stop the release of the clip.