Twilight has learned a new spell, and she plans to test it. The concept of the spell; alternate universes.
She must take one of her friends, she can't go alone after all.
Even though she's never told anypony, she trusts AppleJack more than any of her friends.
So when she does the spell it just happens to take them to possibly the worst alternate universe possible, where alternate Aj is probably the most miserable.
AppleJack wants to change that…which doesn't go so well..