I wondered the same thing the first time I saw that! In English it's rather impolite to refer to your mother (or indeed any woman!) as a "bag". ("You old bag!") This is not so in Japanese! It's actually a term of endearment. Please see this blog post on some explanation.
I'll give a section here in case the link breaks:
I can’t find anything about the etymology of お袋, but I’m going to have
to assume that it relates to the womb as the primordial bag. Just as
koalas are known in the kanji sense for their pouches (e.g., 袋熊,
fukuro-guma: koala, pouch + bear), a human mother is also nicknamed
for her interior “pouch.”
Here’s what Yahoo Japan’s dictionary has to say about お袋. See how much
you can understand before turning to the link for the yomi and
"自分の母親を親しんでいう語。古くは敬称として用いたが、現在では主に男性が、他人に対して自分の母をいうのに用いる。" Yomi and Breakdown of the Words …
A rough translation:
"A word expressing closeness with one’s own mother. A long time ago, people used it as a title of honor, but nowadays a man will
mainly use it when talking to others about his mother."