With Princess Twilight's permission, the Mane 6's and CMC's human counterparts and Spike's dog counterpart took their first extended visit to Equestria. It was a long time coming.

But while the earth-native friends were out chilling/chillaxing or exploring Ponyville with their Equestrian selves, a panicking Princess Twilight suddenly teleported them back to her castle and told them they had to return to their world. If they didn't, existing with their counterparts would risk Equestria's existence by causing a potentially apocalyptic imbalance.

That is if the Princess of Friendship didn't misunderstand something Princess Celestia had told her.