A series of improbable events.
My Missile Frigate with it's 99.2% chance of victory lost all 9 rounds of combat. It only shows 7 because my ship has a first strike (due to the upgraded shields promotion) so it lost a round that doesn't show up at the beginning and it also lost one at the end that doesn't show up because it withdrew. It had only a 5% withdraw chance. So after the worst possible luck in the fight, it gets very lucky and withdraws.
That particular star system also has a rather checkered, and improbable, past. It was a system inhabited by lower tech aliens who, upon meeting my scout ship, joined my empire (one of the less likely outcomes). It started with a population of 2 and no units at all (as is typical). Something like 3 or 4 turns later I was the first to research the tech that founds the Power value, founding it in this star system (not very improbable, it was my only star system other than the capital). Time to build first Planetary Defense Ship? About 10 turns. Time until the pirate invasion ship moves into a visible location 2 plots from the star system? About 5 turns. I did not yet have the tech that allows the civic (or Policy, as they are called in Final Frontier derived mods - foreshadowing by Jon Shafer) that lets you rush buy things (Democracy, or, in this version of Final Frontier Plus, Technocracy would also allow it - didn't have the tech for that yet either). So two turns later, with 3 turns to go on the first defensive ship, the pirates capture the star system. The new missile frigate I sent from my home system upon spotting the pirate arrived 3 turns later (I think it was) to recapture it (good thing it was a pirate invasion ship, not a planetary defense ship). This, of course, reduced it to a population of 1 but did not destroy either of the 2 free buildings the system started with. The missile frigate had to fight off only one other ship before a defender was built (which took about twice as long as the original one would have). Then, somewhat later, the event in the screenshot happened to that same missile frigate.