Ignore me if I'm wrong, but I believe that one reason the ratings of drivers are going down is in direct retaliation to riders low rating Pax low for not tipping. Pax have noticed they're getting lower ratings and they don't even know why. It's a domino effect.
I hear everones adjusting. Supposedly theres no more reeducation classs until 4.4 rating and no more firing until 4.1ish. I havent heard that directly from uber or lyft but thats the chatter here, on reddit and random places.
But asise from that, ive noticed students rate more harshly, and a lot of posts mention that as well... but i dont have definitive proof .