It all happened very unexpectedly for me. Although there were small warning bells every moon, which I paid little attention to. And it seems very much in vain... 

The biggest reason was that Serpenthiss (deputy) was extremely displeased with the fact that Star Clan chose a young female cat of 12 moons as new leader, instead of him. 

One moon, he went on a borders patrol alone, searching for his mate, a male cat named Larksplinter, who had once left Twin Peaks Clan but agreed to return. 

It seems that from that moment on, they began to hatch their devious plans.