Gaius Julius Octavianus Caesar was a physically weak man, whom contemporaries assumed would not live long. At Philippi, Octavian's position was overrun by Brutus, causing Octavian to hide in the swamps to survive. Octavian was not even present during the decisive battle, resulting in his own troops declining to salute him "Imperator" preferring to salute Antonius with the honorific title.
When Antonius led his forces to Actium, he likely assumed that his opponent would be the incompetent Octavian of Philippi and not Agrippa the patient strategist of the Sicily campaign. Antonius likely thought that Octavian will attempt to defend Italy, like Carbo did against Sulla, and be easily defeated. Instead, Agrippa led the Western forces out to meet Antonius.