I'll bet on the following page, the topmost paragraph has only 2 lines in it.
It's the default setting for "widows and orphans", which tells the reader how to break up paragraphs so a few words don't fall on the next page alone as the end of the paragraph. An orphan is the first line of a paragraph that is left at the bottom of the page (or column) and a widow is the last few words of a paragraph that end up at the top by themselves. It's usually preferable to avoid having a single line of a paragraph hanging out alone whenever possible. But that then leaves you with more space at the bottom of the previous page as you've encountered. Some people find the extra blank space more annoying though.
You can set widows: 0 and orphans: 0 in the stylesheet.css for the book if you mind the extra space more than the few words on the following page.
There's some info about widows and orphans in these 2 threads here at MobileRead:
https://www.mobileread.com/forums/showthread.php?t=84469
https://www.mobileread.com/forums/sho...d.php?t=207531