there are a lot of reasons why you get a bounce rate. but first, lets look at the technique: the bounce rate is calculated "bounces : entries", see here: Bounce Rate
here are some reasons why users might come directly to pages which you describe as "non-landing-pages":
- user found a way to access directly either by search engines
- someone posted a link somewhere or sent email to friends
- visit "timed out" (30 mins) because user left computer for a moment, then came back to make one hit (reload?) and disappear...
I suggest you have a look at
a) how many entries/exits do you have on those pages?
b) where did they come from (referrer, previous pagename,...)
c) did the have another visit on the same day?
just a few ideas ...