Cover Art - Coincidence?

While perusing my Goodreads account, I went to visit my "medical issues" bookshelf.  (I have various shelves that describe themes/topics addressed in the book.)  I couldn't help but notice something:

All of these books - the most recent 20 books that I put on this shelf all have covers that contain lots of blue and/or green.  I wonder if there's something about this color that publishers associate with this theme, or if this is all just a striking coincidence.

Thoughts?