There a several circumstances that could come under the rubric of mesh migration:
1. The mesh can fold up, roll up, ball up (meshoma)
2. It can contract or shrink.
3. It can possibly fray and shards from it can migrate to other areas of the body.
The results of the first two can be pulling or impinging on adjacent structures and/or nerves and irritating them. This can be due to breaking free of stitches, tacks, or staples through improper fixation, inadequate fixation, the effects of foreign body reaction, scarification, infection, abrupt movement/trauma/injury, etc.
In the case of item #3 this is also usually due to foreign body reaction, unseamed edges, pulling free from fixation etc.
In some patients, their bodies reject mesh (allergies/infections?) to such a degree that the mesh hardens, blackens, and causes additional problems.
Sometimes surgeons are able to simply remove problem areas of mesh and tacks or staples that are causing problems but sometimes these things have already caused lasting damage and sometimes the entire piece of mesh need to be removed.
This is from a review of the literature but is by no means complete.