Were both cards through the same bank/provider?
If you get a second card through the same provider, they’ll often lower the limit on your first card around the amount of the limit on the new one, because otherwise they just massively upped your effectivel credit limit.
I had an air miles reward card, and then got a nice balance transfer offer for what I thought was a totally unrelated card. I sent in the application, got the card, and saw the credit limit shifts, and that was what I was told when I called to ask what happened. This was like 10 years ago when I’d use balance transfers to keep the debt at 0% (minus the transfer fees, which were trivial then and less so now). I didn’t read the fine print enough to realize that the two cards were from the same bank conglomerate.