Ok. Let's start. Player can decrease or increase attributes based on:
- Rest CA/PA
- Player: Determination, Ambition, Professionalism
- Age
- Coaching Department
- Training Facilities
- Training Schedules
Let's take a close look at CA/PA.
When a player has 100 PA and it's 90 CA; it's highly unlikely to improve in some attributes, like physicals ones.
That's why because player has different weighting for each attribute in each position.
What happens when a player it's nearly to reach PA number or have reach PA number? Player will have slower development or decline.
Why? Because as I said above; all the above factors effect player development or declining.
So, if you focus heavily in physical attributes ("eats" more CA numbers; like development weak foot), have to decrease somewhere else to make room for others.
That's the general idea. There is a lot more details to determine why a player improve or decline.