The instable asteroid field was formed out of the remnants of the planets Kira I, II and III. While the gravity well of Kira kept most of the asteroids contained, a few of them escaped and posed an occasional hyperspace hazard.
When an unknown cataclysm destroyed the first three planets of the Kira system, the remnants of the planets formed an unstable asteroid field around Kira, the system's sun.
This was discovered by a second system survey conducted several years after the first one. Further research into the cause of this event was deemed unnecessary and it remained unknown.
When the Kira Run was established years later, the asteroids remained a small, but constant hazard for hyperspace travel.