Looks like a mode-hop to me. It's probably just caused by everything stabilizing after the initial powerup. Almost all common DPSS lasers exhibit wild power fluctuations during the first few moments after being turned on. Sometimes they mode-hop as the diode comes up to a stable temperature. Probobly nothing to worry about, that's for sure. Just as long as it returns to and stays at TEM00 once everything has stabilized.