Adverse Action
A lot of banks do it when you have what they consider "sketchy things" happening to your profile. Things like applying for too much credit, like multiple inquiries in a short period of time or multiple new accounts. Or something that looks like fraud and etc. Some companies will Balance chance, reducing your credit limit each time you make a payment.....some just lower your limit in general, other banks, like Citi simply closed all 5 of my cards with them, while commenity bank only closed my visa card but left my store cards open. Each bank is different.
