Why is this? Well, from a financial sector standpoint the stock is relatively cheap compared to it's competitors and the only stock trading under $5 with a market capitalization of that size.
From a technical standpoint, the stock is about to skyrocket. That's probably why open interest is in the seven (7!) digits around the 5 dollar mark (Jan 22).
Recently, analytics institutions upgraded the stock from hold to buy. From a fundamental standpoint something must be brewing. For anyone interested in the stock or stock option follow news around Citigroup closely on Google news. I'm probably getting the (5) call option for Feb 19 since it's a little cheaper, today.