This generally happens for 1 of 2 reasons:
 
1) Someone accidentally sent you cards that should have been shipped to someone else. This usually means that someone else is missing cards they want. 
In this situation, open a case and politely message your trading partner explaining the situation. Please click "Request Admin Assistance" if necessary.

2) Someone added extra "throw-ins" to a package they were sending you as a bonus. After checking to make sure this is the case, just complete the trade and say thanks!