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I don't understand how that hinge came loose.

The top is usually held down by the 4 straps, front and rear, and not held down by the hinge.
The strap hold downs are on the inside of the vertical gunwale wall and usually do not pull out either.

I see on your personal page that you so not have the rear straps but instead have ridged uprights which many boat also have. I could understand those rear ones coming loose but not the center ones.