I don't have much experience, I'm lvl 3.
From my short experience if you win matches and don't train your players too much, they don't drop morale or the decrease is slow(er).
If you use players on wrong positions for more matches they will lose morale faster. If you don't use them in matches their morale will drop faster than usual. If your team is dominated and lose badly, most of your players will have a decrease in morale.

Let's hope some experienced guys will come and clarify that for you.