Why is every show randomly getting lower-rated?

I noticed first how most episodes of Little House On The Prairie now are going down in the ratings. This really weirded me out, since I saw no reason for people to suddenly get negative about it. It's not like Game Of Thrones, where people still are upset with the final season.

As if that wasn't weird enough though, I'm now noticing every show has a lower rating than before. No matter how much praise they've gotten.

This is too frequent to be a coincidence. What the hell is going on with the ratings? Is imdb adjusting their voting system?