Why does the game do this?

Laakeri

I was duo with Torarth and we got this killer. I understand game loosens up the rankings the longer someone has to sit in que, but that was just straight up slaughter and no fun for anyone.

Gcarrara

It's basically for the reason you guessed. 🙂

The more the time goes on waiting for a match, the more the rank restriction is loosened.

Enough waiting and you could end up with a way higher rank difference too.