I don't really know why they do it but the same thing happened to my daughter. My daughter works at Walmart and used her card for little things like snacks at work or cat litter, etc. She only had a $300 sl and she used her card a lot and paid it off every month. I have read tons of reviews of this happening which is why I won't even apply for a walmart card even though my daughter works at Walmart and we shop there A LOT. I wonder if it's a Walmart thing or if all Schrony cards are like this. It worries me that they will lower my carecredit before I even use it! I don't know that it was the right choice or not but my daughter got so offended and mad when they lowered hers to a measly $100 she demanded they just close it. She had only had the card for about six months so I think it was fine since it was not one of her over 12 month cards.
They check your credit reports to see if you are late or if you have any other issues with your other accounts. If so they will decrease or close your accounts. They do it all the time. Google Synchrony Bank reviews...There are a lot of people complaining about the same thing.
Synchrony will decrease you all the way down to $100 if they have to because they do watch your credit reports. Last Jan. my Amazon store card was dropped to $100 and it took me awhile to get it back up to $4000. My old limit was $750 or something.
They did this when I had 5 accounts get 30 day lates and 2 accounts go to 60 days late. I brought them current but they still decreased the limit.
Do you have other accounts that are late or in collections? You mentioned an amazon card in collections. Is that credit score a current FICO 08. That could be why they keep decreasing your limits, because you are high risk of default.