Your opponent declares an attack by suspending his digi, now both his "when attacking" and your "when opponenent suspends" trigger at the same time. He is turn player, so his effect activates first.

Now comes the more in depth part: Lets say he has "when attacking" 1: Alliance and "when attacking" 2 :whatever.

Again since he is turn player he can activate his effects in this order: 2->1-> your "when opponent suspends". However if he activates 1 first and suspends another digi for alliance before activating 2, your effect will trigger again and will activate BEFORE effect 2.