That might be the problem. We put little stickers on the lens hoods of our big Canon and Sigma lenses, warning customers not to over tighten them.
What's going on with these hoods is that there is a liitle component like a chisel blade (though not sharp, obviously) which makes contact with the rim of the lens and holds the hood in place. So you have this blade on the inside of the hood, and the knob on the outside of the hood, and the only thing that keeps them together is a tiny little circlip on the spindle of the knob. Tighten or untighten the knob too much, and this clip gets forced against the blade or the inside of the hood, and it pops off. (And it's so tiny, you'll probably never find it.) It's very annoying, and I would call it a major design flaw if I could think of a better solution, but I can't.
If I were you I'd secure the lens in place with electical tape, or failing that gaffer tape. It won't hurt the lens.