During the printing process, water gets removed from the paper as the web moves through the dryers to dry the ink. The water content in the bundled sigs stays relatively the same until the binding process.
In the gathering / binding process, opening the bundles offers the printed stock its first real opportunity to start taking on moisture. Once the books are trimmed, the paper will still continue to absorb moisture, which lends to paper growth. Since paper has a memory regarding its moisture history, it has a natural tendency to migrate back to the water levels it had when it was produced at the paper mill.
Sig growth or paper growth is most often noticed on softcover books since the cover and text are trimmed to the same length. Paper growth is also dependent on the paper stock being used. Coated stocks are less likely to have growth after a book is trimmed. You are more likely to see growth with groundwood and some uncoated stocks.
All print companies experience this issue.