I hate to say it, because I think people often overvalue it as an explanation for D&D's recent success and this suggests that I'm wrong about that, but March of 2016 would be the 1 year anniversary of Critical Role's first show.I am kind of curious what happened in March 2016 that stabilized the position. Were the other dips because it went out of print, maybe?
However it might also just be when Wizards finally figured out that 5e wasn't following the usual sales trends on core books.