Since there really is no grease used in these units where it could get on clothes it's probably residue that has built up in your drum. *Scrud*

Set your unit for its hottest wash and add a full cup of OxiClean to the wash tub. This may need to be repeated twice but in most cases this will clean out your washer.

As to why this happened there are many posts in G/W regarding this but mainly it comes from washing with liquid detergents and also washing with mainly cold water.
All washers, Top Load and Front Load should have regular hot water washes (at least once a week)to keep scrud from building up in them.