Troop morale

hi ok im getting confuse as to why aircraft in particular loose morale for no apparent reason. Just started a new game sent 2 fighters and a bomber to attack 3 light tanks no issues sent em back again returned doing no damage as unit had moved out of range morale still at 100% on return. Split units send bomber alone notice its now at 90% upon departure checked the fighters they at 95% tanks obviously dont carry anti aircraft guns and ive never known one to shoot a plane out of the sky so can someone explain whats happened please. Thanks in advance