I can only remember one exemple like this happening to me: in cup semi, I think, oppo slightly weaker, I won first leg 3-0 and could have been 5-0 if I well remember. I was overconfident and did not watch second leg. Got destroyed 7-2. But your case is worse as you were watching and his tactic was just silly. After many seasons, I also noticed that the game engine has a tendency to favor the "best" team in cup. If you lost first leg badly against all odds, the chances are you will come back on second. Did it several times. But in your case, that does not seem to apply.