Thanks everyone for the reminders and ideas!
In the one we have going, some of the PC's are actually the slaves. Some of the other PCs are the guards that are going to free the slaves.
The ideas I'm thinking of are:
1) the guards (some) had known the slaves, prior and are freeing them out of friendship.
2) the guards were hired by an outside agency that is trying to prtiect the slaves
3) guards are hired by an unknown organization and the slaves also do not know either the slaves or the organization that is backing the attempt. (My favorite, since it can lead to other events down the line.)