Excellent guess! When a boid hits the boundary of the sphere, it picks a new target position on the opposite side of the sphere and rotates towards it. This leads to some boids turning left and others turning right and they have the potential to break off from the group. But if the bounds are turned off, then using the settings you had, they would just swim in a tight group forever. The bounds keep things interesting :)
Thanks for checking it out, glad you enjoyed it!