After jumping out to a 21-0 lead just after the 1st quarter ended, the Bengals defense gave up 3 touchdowns before the half.
They were not able to pressure quarterback Philip Rivers on a regular basis, and he completed 14 of 21 passes for 235 yards and 2 touchdowns in the 2nd quarter alone.
The problem was not playing zone defense. The Bengals were actually very balance between zone and man and over the course of the game they gave up similar numbers in each.
Teams will generally play more zone when they have a big lead.