Riot, can I get an explanation about my MMR and stuff please?

I just got to demoted to plat 1 from diamond. I thought we were supposed to get a warning letting us know we were close to getting demoted? I thought we were supposed to drop a full tier lower than our current tier/division (so plat 5 mmr for me). I checked my previous 10 or so games plays and the average of everyone seems to be around plat 2. I am confused how I got demoted so quickly and without a warning either. And now I am losing 25 points per game while gaining only 9. I don't get it. Like maybe it's "working as intended" but can someone explain the intentions here? It just seems so random. I didn't get the notification or reach a full tier lower. What's going on...?