I would suggest a reason of my own for why this might have happened.

Certain players can get tired more rapidly during a match based on what they are doing during said match.

e.g. (From personal experience) Take for instance that your wingers are trying to push the flanks. Say the AML (I love me some 4-5V-1 action <3) continuously acquires the ball and goes for the attack as you have gone for flank focus. Well if the DR/DMR is worth his salt he can definitely tire out your attacker if he is able to constantly intercept your AML.
And this can be done by a single player, it isn't needed for your opponent to put the pressing style in to HARD. If an individual player has to work harder to make his intended actions a reality, he'll put more effort and... well, stamina in to achieving his goals (No pun intended).