I been noticing this also. Recently I added 20 new pages on a very old trusted site that ranks very well. All pages were indexed 2 days from addign to site, 3 days later 10 of the pages fell into supplemetnal. The pages are well linked, unique content, unique title and Meta tags. Where usually any page that I add on the site gets indexed and ranked within two days these pages fell into supplemental.

Waiting it out to see if it returned to normal index.