This theory is based on my one-time experience and resultant feelings; nonetheless, I suggest that it makes some intuitive sense.
It is very, very sad when your father dies, or your sibling, or your friend, or your dog. When your mother dies, there is this additional aspect. So hard to describe, but let me try: it must be like the anomie of uprooted peoples. One becomes unmoored. It brings a whole new and serious meaning to "Who You Gonna' Call?"
Your mother's death turns the life experience from one of continuity into one of acute awareness of its finite nature. With your mother there was no beginning, none at least you were aware of. And then, more than any other in the entire world of your existence, your mother is Just There. Not even thought about as being there. "Where's your mother?" "I don't know [but I know she is there]." There is a tether, literal at conception. It's there still, if figurative, afterwards, felt at lesser or greater degrees at times and for individuals. You're at one end. She's at the other. Omigod, now she's not. You will never get back to the Mothership.
To be practical, nothing much changes in day to day life when you're an adult and your mother dies. You can pretend she's back there at home and you haven't called in a while. But something else changes; something in the world is gone. Don't look around that void too much--it may be that that's all there is.