Well, that is default behaviour, to stretch space on some places (before/after headings, between paragraphs, in lists etc. where it's possible) to fit the page vertically, i.e., to get bottom lines even on all pages and get "squared" design. You can easily remedy this by placing in the preamble the following command:
But, in this case, expect, well, ragged bottoms on pages

Since you'll probably have to manually sort problems with ragged bottoms, it is very likely that you'll want to add/subtract a line of text on a page from time to time, and for that the following command can be useful too:
Use this command on pages for which you want text height to be one line larger than normal. You may use negative values for the opposite (e.g.
\enlargethispage{-2\baselineskip}).
Typographically speaking, it is desirable to have squared pages, but without widows (orphans may be tolerated). To achieve this, there is no other way but to do manual corrections. This means rewording the text, or creating manual breaks and/or enlarging page spreads with the above command. The method I personally use (in case rewording isn't possible) is to create custom line breaks on some places where it is possible.