Chears mate, something similar happened to me, too. 4-1 loss at 13.3 (me 16.7). Match was even (me +6 possesion, but equal on chances) till of second half, than I scored and without any changes (other manager wasn't there) the match totally turned. Equalizer and then 3 fast goals. He probably used a good counter tactic or the engine sucked.
Well, I will use the second match to expiriment and try a different formation (4-2-3-1 (me) vs. 4-1-4-1 was a fail), will try 3-2-2-2-1 in the return leg, but I don't expect to turn the result.