With every League promotion, a team is playing on a more advanced level, and players need to stay on top of their game and training in order to develop their skills further. With that, lost stars represent a visual adjustment to a new competitive environment. 

As you promote to the next level, your player's stars get adjusted to reflect the new level of play. This is why it may look like your players have gotten weaker with their OVR decreased, but the Total Quality of players has remained intact, together with any additional Tier increase that can be added to the players’ OVR.

As in real football, a very good player from the second league may just be an average player in the top league at the beginning. This means that stars are linked to your level: The higher your level, the more quality/performance is needed from your players in order to stay competitive.