It didn't crash per say. It was coming here by choice. And yes it can travel back and forth. It is actually better to say it exists in both places and all along the space in between. The best way to visualize it is exactly as the book describes it. Outside of everything there is a cosmic barrier which keeps the Deadlights at bay. They flicker and swirl and constantly push against this barrier. Every now and then a beam of that unholy light finds or makes a tiny opening. It isn't enough for the Deadlights to come through entirely, but it shines through just the same. Think of it as a laser cutting into our world and burning a hole into our reality. The point of the light's impact was Derry in the "ago" and it has been here ever since. That light is shining through the hole. Because the Deadlights are mad and flickering in a pattern, sometimes the light is dim and sometimes it is bright. When it is bright every 27-30 years or so, the creature is active. When it is dim it sleeps. To exist in our reality it had to take physical form so at this end of the beam of light, there is a monster. At the other end there is the Deadlights. There is always a connection between the two. That is why the physical body of the spider-thing their minds conjured up was still there even as the soul of Bill and It skated off across reality toward the outer dark.
And right there you have your answer about whether or not it can come and go. Of course it can, because it exists in both places. It is going back to try and toss Bill out there.