The mother needs to stop putting words into the child's mouth - bet she nods her head at both of mom's suggestions of what is wrong.

It could also be anger at mom which is very common in kids that have been in daycare all day in the sense they are so relieved that mom has come back they get their emotions mixed up and will hit and act up instead of hugging or hug and then hit or whatever odd combination they come up.

As for it being a stage there are times when they develop their depth perception, times when they are afraid of heights, etc. Maybe mom is holding onto her too gently and she needs a firmer secure hold. Also they grab on to whatever is there and their small hands can only grab the skin at the back. For some kids they grab your collar and strangle you or worse the back of your head and pull your hair. I have always thought it had to do with feeling secure while being carried since that seems to be when it happens.