Sounds like you're using the "Adobe Paragraph Composer". This optimizes
the spacing of words throughout the paragraph, avoiding wide word spaces
on one line, and narrow spaces on the next. To do this, it sometimes has
to readjusted the line breaks right from the beginning of the paragraph,
and that may be what you're seeing.
To avoid this, from the paragraph palette, choose "Adobe Single Line
Composer". This will do the justification line-by-line, without looking
at what came before and what comes next.
However, be aware that unless you're planning to spend a lot of time
lovingly adjusting the line breaks in each of your paragraphs, you'll
get much better spacing by using the Paragraph Composer.