If you haven't unlocked left justification on your Kindle, you connot have done this yourself. (or at least I connot on 3.0.3) Some books are simply formated with left or center justification and sometimes even with the justification unlocked, you connot change this without reformating it first. Also, if it's DRM'd you connot to so without removing the DRM first.
You did say that at first this particular text had left justification. It might be that the pages before this particular point in the book was indeed left justificatied, but then it might have changed to center justification at this point in the book. Try to go back in the book and see if this is indeed the case. If so, then it might be bad formating on the publisher's side.
Also, try to unlock justification on your Kindle. See link below:
https://www.mobileread.com/forums/showthread.php?t=99661