It just so happens that this is the only time of the year that one can get Coke made with cane sugar - the yellow Coke caps indicate the kosher for Passover Coke, which do not use corn syrup.
Coke aficionados have known for years that the only way to get Coke made the old-fashioned way with sugar was to buy the Passover formula Coke, and they probably buy more of them than Jews do. (Mexican Coke is apparently also made with sugar.)
Pepsi seems to have noticed this trend, and it is unwilling to concede even this limited market share to Coke. So, Pepsi introduced Throwback - this is its second year - at roughly the same time that Passover Coke is in stores.
The ironic part is that Pepsi Throwback is not kosher for Passover!