Why do some people pronounce 'a' as 'u'?

Solution 1:

After having read the comments above I'm going to take a shot in the dark and assume that @Cass73 is referring to the way that some British accents may pronounce an 'a' in a similar manner as the 'u' in crumb.

The short answer, as @Mitch said in the comments is that questions like these are difficult, if not impossible, to answer definitively. It's the way that people from a given region pronounce these words in this way. Whether they changed, or people outside that region changed, or everyone changed from the way the words were pronounced canonically 200 years ago is something you would need a linguistic historian to answer.