Flash Going Crazy!!!
This is a "feature" Canon incorporated into their speedlights years ago so you can use the xenon light as a preview modeling light. Only they didn't seem to bother to tell anyone. This used to drive me CRAZY--I'd be shooting a photo at some gala and the bloody strobe would blind my subjects if I brushed against the depth of field preview button.
Dig out the flash manual--you can go into the flash settings and turn this stupid feature off!
If you had read the manual fully in the first place you would have known about this.