Why does An-225 have anhedral wings though it is a cargo aircraft?
Anhedral wings will induce roll instability and improve roll maneuverability. 

In a large/heavy airplane with a high-wing configuration there is usually excess roll stability, so this type of wings can be pretty common.

Both the high wing configuration and wing sweep contribute a negative sideslip-induced rolling moment, and anhedral is necessary to limit this moment. If sideslip causes strong rolling, the aircraft will have a tendency to dutch roll.