My pics have been rejected because the category is full, although it didn’t say so when I uploaded them, why is that?

Our PMs manually check all the pictures submitted by our OneFormers. Moreover, we work on a first come first served basis. This means that if two pics are seemingly submitted at the same time and both of them are valid, but there’s only one spot left, the one submitted a nanosecond before will be approved, whilst the other will be rejected.
We are working to improve the system every day, so the categories close as soon as we reach the limit and we try to ensure that this situation is as uncommon as possible, but if this happens to you, this is why.