I actually remember the very first time I heard the 'your mum/mom' thing. It was in primary school (8/9 years old) and my mate kept saying it to me one day. He'd met my mum loads of times and I didn't understand why he kept saying it. Eventually I asked him and he replied that it was the new thing to say.
I went home wondering why my mum was the new thing to say. Very strange day so it was.