the pressure forces quicker throws. This cuts down on errors because the DB's don't have to cover as long. Last year zero pressure means more time in the pocket for a QB which = DB's having to extend their coverage.
I'm not making excuses for poor mental decisions on the part of our DB's but it's important to realize the risks our DB's have historically taken (due to lack of talent) are exposed to a much higher degree when the QB has time to throw. For example: If an out route that our DB's have always liked to jump (risky!) turns into an out and up, the DB is toast. Decent QB pressure goes a long way in removing the ability for an extended route to develop which is why risk has worked in the past.