First thing is you need a DUNS number, which is a credit account for your business. When you set up an account with them you would be asked to list your vendors with which you have payment terms. The DUNS organization will then contact them to verify your information.

If you are the person responsible for paying your business expenses, then they can look at you and the business as one.

Still, kind of a dirty trick to cut your limit so far without advance notification.

Most likely you gave Citibank your SSN and they used it to check on your personal credit history as most credit card companies do.