Thoughts becoming reality is what the Traveller used to take them there in the first place.
This is an ability his people have, which is what enabled him to move the Enterprise so far and so fast. After he catapults them across the universe, they arrive in a place where the difference between thought and reality is very thin, because he more or less identified the two in order to get them there.
From the episode's transcript:
TRAVELLER: You do understand, don't you that thought is the basis of all reality? The energy of thought, to put it in your terms, is very powerful.
KOSINSKI: That's not an explanation.
TRAVELLER: I have the ability to act like a lens which focuses thought.
KOSINSKI: That's just so much nonsense. You're asking us to believe in magic.
TRAVELLER: Well yes, this could seem like magic to you.
PICARD: No. No, it actually makes sense to me. Only the power of thought could explain what has been happening. Especially out here.
TRAVELLER: Thought is the essence of where you are now. You do understand the danger, don't you?
PICARD: Chaos. What we think is what happens.