If your reserve roster is full (can't remem what the limit is).... all other cards auto go to the inactive section. Should always check that as since this is still beta, there could be a bug.
Inactive is needed for those that want to stock pile/collect card as well as other reasons.
If you didn't have this feature, you'd have to get rid of cards before you could accumulate more. There's no limit to the amount of inactive cards you can have.
