Probably you are using
\flushbottom or a document class that uses this as default (
This usually happens, if there are large objects (figures, images, tables) that do not float and TeX cannot break them accordingly. Then the large object has to be moved to the next page and the previous page is stretched to obey
If you have many non-floating objects, then
\raggedbottom helps. It probably hurts less than the overstretched pages. But consider floating objects (
Then the text area isn't too much interrupted, it is easier for TeX to break the pages and
\flushbottom can be kept that means opposite pages have the same bottom border.