It's not exactly random, there is a hidden factor that decides how many condition is lost during activities.

When a player is in bad shape (or in my words, "poor motivation") he will lose less condition and often result in poor performance in matchs.
In general, good performing players lose condition faster. If you take note on the condition loss during training you can even figure out if a player is in bad shape as he will lose condition much slower compared to his peers.

If you are desperate to use this player, spend 2~3 green packs per day and he will get back to shape rather quickly.