Super-Kamiokande is a cylinder for engineering purposes. It is about forty-one meters high and thirty-nine meters in diameter... holding twelve and a half million gallons of water! Blasting such an enormous cavern underneath a mountain is not possible to do [safely and stably] for a sphere.

As for the event itself: The first two particles are emitted back-to-back in the rest frame. To our eye, they are boosted and look like they are going somewhat in the same direction. The other ring -- and I am not sure what caused it -- is indeed going in a very different direction to the first two.