ok now that is because you actually didnt have a fleet
and then when you were attacked the 2nd time, you got a lucky draw and your slave master`s ships defended you
(when enslave a master can choose to send reinforcements when slaves are under attack, however it will be the 3rd or 4th stack if they have one, and there is only a 1/3 chance you will get it)
but that is basically what happened there