How in the world can it fall out of the tube and wind up in the abdomen?
It happens when there is a structural problem in the mother's tubes (like a hole or the end does not attach properly), and is more common than you would think --- usually just resulting in repeated miscarriages (or failure to fertilize).
It's nothing odd sexually going on, if that is what you were thinking.
No, that wasn't it, LOL. Female plumbing is complicated enough as it is, and I wasn't sure that I wanted to know how an egg could get from the tubes to the abdomen. Sounds reasonable enough (I guess).