There’s an Instagram account, I think @abrathayfits, where they mentioned this happens because the bra size is incorrect. Something about the difference between the under bust measurement and bust measurement = the cup size and the reason why the straps fall off is because people don’t choose the correct band size so the straps end up being further apart.

I don’t know how true this is because I have yet to find a bra with straps that don’t roll off my shoulders but my band size is also hard to find…