I have Alma bb epi electric, daughter has Alma bb damier ebene. I know exactly what you're talking about, and I think I can tell you what causes it --- at least on the Alma's.
What happens (at least in our case), is that it depends on the direction that the strap's clasp is hooked onto the triangular "hook". If it's in the wrong direction, the rolled handle, when lifted, will open the clasp.
Let me put it this way .... if you look at the clasp at the end of the strap, it's sort of like a lobster claw clasp, right? That is, to open it and hook it onto something, there's a "tongue" that bends inward by spring action. Let me call that the "tongue". (I'm speaking here specifically about the clasp at the end of the shoulder strap. Sorry, it's hard to explain with words.)
Then there's the metal, "triangular" hook, attached to the bag's handles, where you're supposed to hook the strap's-clasp onto. Ok, well if you attach the strap's-clasp to that "triangle", in such a direction that the "tongue" part of the clasp is facing outward from the central zipper .... then when the rolled handles are lifted up, part of the rolled handle actually presses against that "tongue" of the clasp. This pushes the tongue inward, inadvertently opening the clasp, and now if it's bumped in just the right way at that point in time, the clasp just slips right off the "triangular" part.
If that doesn't make sense, look at your handbag. Re-attach the strap's-clasp so that the bendable "tongue" is facing outward and away from the zipper. Now life that rolled handle. See where the rolled handle presses at the tongue?
The solution is just to unhook the clasp, turn it around, and re-clasp it so that the bendable "tongue" is facing
inward, toward the central zipper. Like the OP said ... it's "hitting just right and opening". Just turn it around and it'll be fine.
Believe it or not, that was the very first thing I showed our daughter when she just got her bb recently.
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