Diplomatic loss would only occur if you had diplomatic victory on, and it would require a certain number of diplomacy points which are accrued with the diplomatic cruisers being in another players gravity well. A Normal defeat is achieved by losing all planets, or in the bots case it surrenders when it's economy goes under from loss of planets and ships. There must have been another condition set such as occupation, research, or capital ship. If you have a save check the option before you push load to see what it's set as.