What does 1 Kings 16:15 mean?

Zimri became king in the twenty-seventh year of Asa’s reign over Judah, but he only lasted seven days in Tirzah. The army was camped nearby, attacking the Philistine town of Gibbethon. 1 Kings 16:15 - Modern Text Bible

(Zimri’s reign is incredibly short—just a week. The army is away fighting, which leaves the capital vulnerable and Zimri’s position unstable.)