You can understand "back" as a verb "back up" and add into: Back up into
You might think that sounds strange, but that is correct to understanding it.
First, you might want to find definitions of back, verb. If that doesn't help, you can search for back into. The advantage of the Internet is that you can put quotation marks around just about anything, then search:
define verb "back into"
Here, I saw back into in bold, so, under back, verb, I saw: — back into: to get into inadvertently (well, that's one definition, and comparing it to your title, it seems to work.)
But here it was perfect: to move backwards, or make someone or something move backwards; back into/out of/away from etc. She backed into a doorway to let the crowds pass by.
In addition, it gave me a few more phrasals using back. :)
By the way, that book looks really interesting, and I like National Book Award winners, so you have helped me, too.