Shrike never received his nine lives because he was too afraid to receive what he thought might be StarClan's judgment for his deeds. He lies to the clan and takes up the name Shrikestar as a falsehood. Whether as a coincidence or from suspicion, Hay and Flare find out and (maybe Bean helps/does a lot) corner him and force the truth out of him. He either runs/is chased, or is injured and his body dumped in the outlands. In any case, he is driven out. Hay announces to the clan that Shrike was a traitor and lied to all of them (actually true perfectly enough) and takes over leadership.

Flood stepped down rather than died as leader, so some look to him to take back over once Shrike is chased out or at least appoint a new deputy to become leader. But others think that since Flood appointed Shrike it's a sign of his poor judgement so the Clan is split between Flood and Hay. It's decided a physical fight happens, and Flood is killed by Hay since he's very old by this point.

Flood was quite happy for everyone to have their secrets, eg dancing and her kits, cloudy and ellie coming into the clan, letting sorrel and ivory stay, stuff like that. he was not someone to insist on truth if it threatened the peace. So as much as that worked while flood was leader, it would have left a minefield for shrike to navigate since there were so many secrets under the surface. it just happened that his was the first to come out.