>>>Why did they not tell us or my mum about this?
Probably because they could see little point in worrying you about something that neither you nor they can have any control over.
>>>How do they make such an assumption?
Without knowing your mother's medical history we can't answer that. However some things are known to increase the risk of dementia (such as suffering a stroke or an inability to properly absorb B vitamins), so perhaps they've seen a high-risk factor in your mother's records.
It's also important to note that the reference to 'dementia' might not relate to Alzheimer's Disease, as there are other forms of dementia:
http://www.nhs.uk/Conditions/dementia-guide/Pages/causes-of-dementia.aspx