It is a question of emphasis. The emphasis is on the subject, "autók", not on the verb. Therefore the verb gets split.
Q: Who comes out of the forest?
A: Cars do. Cars come out of the forest.
If the verb is not split, then the emphasis is on the verb. But then you need the definite article in Hungarian:
"Az autók kijönnek az erdőből."
Q: What do (the) cars do?
A: They come out of the forest.
Interesting, thank you. I did think it had something to do with emphasis, but it's hard to tell sometimes. I have it in my head that the preverb is attached to the verb in front, unless it's a question or negation, but that seems to be an oversimplification. For example, in another exercise, the preverb and verb were not separated even though it was a question. But the sentence didn't have a question word, so that wasn't necessary. I'm starting to get the picture now of how big a role the main verb has in a Hungarian sentence.