To actually answer your question; salary drops during the offseason of each season. During the offseason, new salaries are calculated based upon the (lower) skills of older player and possibly a change in salary formula. So the salary of your player will stay the same until the end of this season. His skills will drop though, with possible drops every week.
There is little you can do to prevent this, you could get a trainer with a specialization in career extension to slow the drops a tiny bit, but best would be just to live with the fact that his skills will drop.