He had prolly setted the dmc with a down arrow, turning his formation in something like a 4-2-2-2. You should post match stats so that we might understand where he broke through your defence and how did his own defenders block yours. Anyway yours wasn't a winning formation, quite normal that the game turned out quite open to any result:
his AML-AMR-ST-ST against your defence were 1vs1, assuming that your DMC was up-arrowed. Or 4vs5 if it wasn't.
You had AML-AMR-ST against three defenders, 4 if the DMC was down-arrowed. Same as before.
MC-MC against MC-MC.
....and thus the smallest detail(luck asw) made the difference.