SBMM at it's finest
First few games went pretty well, most survivors matched my skill level, I even managed to 4K a couple games. Then SBMM kicked in. And I got this.
They were clearly working together, nobody stayed on hook for more than 10 seconds, they were constantly bodyblocking to take hits, and the exit gates were open in well under ten minutes. I got lucky and managed to get a single kill in the gate, and only because I brought a Mori to complete a challenge.
I know rank doesn't necessarily match skill anymore, but come on. FOUR rank 1s?! After two or three games where I 4Ked with bronze and brown players. That's an insane jump in skill level, and they obviously ran circles around me the entire time.

Comments
It looks like you're encountering what I personally have encountered. If there's not enough high-ranking players currently online within your region, then most likely you'll be playing against people you have no business playing against. In a way it's no different than the old style matchmaking. You'll get some good matches and some bad matches. But if it's any consolation, according to the devs you just lost four straight games, so your next game/s should be easier.
Here's the thing: Grade doesn't correlate 1:1 with game-based skill, but it DOES reflect 1) how many hours they've put into the game over the last 30 days (or in this case, 20 days), and how reasonably successful they've been over those games (as ranking up still requires pipping, which still requires high performance).
So yes, you're not necessarily playing against a Tournament Squad just because they're all Red 1. But you ARE playing against a group of people who've been playing the game so much in the last 20 days, and so well, that they've hit max rank with a third of the month left to go.
That's 90 pips. Even if they got 2 every game they played, that's AT LEAST 45 overwhelmingly successful games, in around two weeks. That's a LOT of DbD!
So yeah, Grades DO kinda reflect skill level.