There could be a number of reasons:
1. Your player is now out of contract, but is still playing for your club. The club is trying to buy him on a free transfer because you have not offered him a contract. In order to check this, go to the Player's stats, then click on Information and check the expiry date for the contract. If it says N/A, then you should offer him a new contract, so that he will not be allowed to go for free.
2. The contract of your player has become unprotected. This is a new feature for Champ Man 4 series, whereby 3 years after signing on to your club, any other club can offer him a contract without asking you. If he agrees to go to their club, however, they have to pay you the entire sum of his remaining wages (I think) in compensation. To avoid this, offer the player a new contract every now and then.
3. The player is under 17 years old. With these you can do nothing, but watch them leave your club if they so wish. All you can offer a player like that is a Youth Contract and if another club offers to take them and they agree, then it's tough luck.
That's basically why someone may be able to get one of your players for free. Ask if you want to know something else.