James, welcome! Thanks, on your Mum's behalf for asking....I do not know the stats but it is very common to get called back for a second view. It often is nothing more than a shadow or perhaps movement. Or sometimes there is an area that does not show up well and they need to get anotehr look from another angle....and yes, sometimes it is something that they are concerned about
and need to review.
It is scary for her and for you, but take comfort knowing that a huge number of women are called back routinely...and if they see a lump, 80% are benign.
You may offer to go with her so you can ask questions of the medical staff. But otherwise, just be there for your Mum....