Hawks Defense Scheme Question

Since the 2015 season and even more so this season Seattle has had some noticeable holes in their pass defense/ secondary. From 2012-2014 the Hawks always held opposing qbs to low completion percentages and dismal passer ratings however this year opposing qbs seem to have an easy time making a high percentage of completions.

I just want to know what the causes are of these struggles in the pass defense. Is it a result of players struggling individually or does it have to do with the defensive zone scheme? I assume Kris Richard is a good DC however I am starting to question his coaching as in the past few weeks qbs are throwing the ball successfully almost at will. Seattle typically plays zone defense and can someone who understands football from a coaching standpoint explain why they don't play man to man and if this zone scheme is the reason they are having difficulties defending the passing attack.

Thanks!
Sully.

Kam and Bennett are both out with injuries, with Bennett having even more impact. That's a huge factor, via cause and effect alone, before even mentioning the CBs. Also seems as though Lane has either lost his mind or can't function without the DLine bringing substantial pressure.

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Lane, who seemed to be the perfect nickle CB for a few years, can't seem to get anything done on the field except cause penalties, whiff on tackles, or give up too much cushion on the receiver.