Congrats! My grandmother would probably say "don't look a gift horse in the mouth" or some such. In other words, don't ask questions. Just take the generosity and don't give them a reason to change their mind. 🙈
I think you were sort of low-balled on your SL to be honest, just based on your scores and getting the minimum $5k SL. No doubt your first cycle spend nearly using the entire limit helped, but I think that the fact that they started you as low as they could on that card played a role here in you grabbing that auto CLI.
I had a similar thing happen. I was approved for the CSP card in mid March, 2018 with a starting limit of $5,000. Within the first month, I put about $3,000. on the card, and about $1,000, the second month, and less than $500, the third. I'd paid off all but about $1,200 as of early June. Toward the end of June Experian notified me of a change to my credit score, and when I reviewed the change, I discovered Chase had doubled my limit to $10,000. I thought it must have been a mistake, so I logged into my Chase account, and there it was, the increased limit. I don't know what I did to cause the increase, but I'll take it. And congratulations on your CLI, too!