Sry i have never seen my urchin do that when i had one. I found this article online maybe its what your urchin is doing.
Reproduction
Longspine urchins reproduce by spawning or releasing eggs and sperm into the ocean at the same time. Females can produce millions of eggs each time they spawn. Since the egg and sperm are
broadcasted into the sea,
fertilization or the joining of egg and sperm to form a tiny urchin, is most likely to occur if a large number of
sea urchins have gathered in the same area and spawn at the same time.
Tim