Not particularly investing related, but I'm sure it was intentional. It happened with every card I've had.
I can't tell you for certain their requirements, but I never missed any payments, and had them all for years. There was a job change and relocation that came up, so I let myself carry a balance for a few months and any time I got with a few thousand of the limit, they upped it on each card. I imagine they only so it for those they find responsible, and if I carry a higher balance for a bit, they make more money.
What I do know is that being over 30ish % utilization is negative credit wise, so a raised limit to not have as high of a utilization is also a good thing.