if you change the players training schedule, his stats will drop first before improving (if you change him from Target Man to a Poacher schedule)
they will drop if you ask him to learn a new move as well as a new position, you will see in the training tab that 20% of his workload is focused on him learning the new move and position, meaning he will only be on 80% of the training, hence is stats will drop.
it happens mate, off season, maybe upset with something, injured, different training sets for different positions require different stats hence maybe shooting more and loses 1 or stats in crossing but as hazman says above, he will come back strong in about 1 -3 months and the position specific stats will rise quickly so dont stress <)
if you buy a player from a club that has better facilities than you do, his stats will drop!
if you retrain a player in a different postition, his stats will drop before rising again!
if you change the players training schedule from winger to attacking mid, his stats will drop before rising!
please search next time, these are all from different threads asking the same question I have replied too in the last several months