ok, so ZLO went hands on and made an explanation :
so I click engie, bomber drops bomb, I move mouse to other engie press shift click on it, in that instant engie dies and I continue to click on other engies, that instant when I press shift, the bomber retargets the mass extractor, so in his queue, the mass extractor is there because of the milisecond that I take to press shift and click on the other engie