Are you asking if it occurs on that scale once a year?
I've never seen it that large, but I would think it happens every time a school of shad gets that large and a large enough school of sandies finds them.
I don't know if it happens all summer, or only in the summer, because I used to only fish during the summer. Someone with more knowledge than I would have to inform you of that.
If you're ever out on the lake and you see the birds swarming a spot on the water, you know there is some type of fish below it. I'm surprised you didn't see any with the scale you're describing.
Some of my fondest memories of fishing are of my father and I getting skunked fishing for crappie for hours(or as I call it "fishing with dad"). We would race across Ray Roberts to find the shad boiling, or looking for the birds feeding on the shad, stop outside of them and pull up a fish every cast.