You do not say what type of cactus it is that you have, but from
the description I would venture a guess that it is an epyphitic type.
If so, "air-dwelling" plants develope aerial roots, usually at nodes
along stems. This is in response to a need for moisture and nutrients.
Perhaps this is the case with your plant, even though the roots are
coming from the middle of the growth. Hope this helps.
WILLIAMSONCA at PHIBRED.COM