Unfortunately when you have metal bits flying around, there is no “usual suspect”, nearly anything can happen. Having said that, the lower rod bearing is something to be replaced “just because you can”. The cage will eventually fracture on it’s own, I sort of doubt if it will swallow metal bits from outside of the bearing, since it is spinning around trying to throw stuff away from itself.
If you had a real rod failure, there would probably be a hole in the case somewhere…although the big end of the rod will start eating at the cases, right where it gets both halves at once, when there is enough rod looseness. That usually doesn’t happen if you replace the big end bearing at a reasonable schedule. Any trace of bluing from heat and it’s too late, you need a new rod and crank pin both.
So the moral of this long-winded story is that tear down and close inspection is critical to keeping these little monsters alive. Clean stuff like you mean to bake on it, so you can really see the condition of the metal surface.