What made the discovery of O-My God Particles a shock to astrophysicists?

On the night of October 15, 1991, the “Oh-My-God” particle streaked across the Utah sky. A cosmic ray from space, it possessed 320 exa-electron volts (EeV) of energy, millions of times more than particles attain at the Large Hadron Collider, the most powerful accelerator ever built by humans. The energy of this particle (probably a single proton) was equivalent to a 142-gram (5 oz) baseball travelling at about 26 m/s (94 km/h; 58 mph).