There are three answers to this that I'm aware of...
One is that it's unintentional and occurs when someone is talking to someone who reminds them of their mother. This is usually down to the situation they're in, where they're talking to a woman who's caring for them.
The other is the intentional use of the word. In some regions (I can only talk for the UK), it's common to refer to older women as "mum" or "nan". It's an affectionate term without any massive significance, which is used quite frequently. In many ways, it's like calling someone "dear", without the patronising overtones.
The last one is that it's used sarcastically, to suggest to a woman that they're being overly solicitous. For instance, if my wife were to remind me to wear a coat when going outside, I may say "Yes, mum". And then roll my eyes. (As long as she can't see me)