Possible reasons for accelerated email list decay?

One of our regional offices is experiencing a list that is shrinking at above average speed.

Started with a total of: 25264

June 2016: 21005 mailable leads

November 2016:     18109 mailable leads (14% decay)

July 2017: 13708 mailable leads (24% decay)

Overall Yearly Decay: 35%

Benchmark: Yearly Database Decay = 25%

Right off the bat I am thinking bounces and unsubscribes, which I'll try to run some reports on. But was looking for other ideas (also suggested ways of running reports like this to do a suitable comparison)

Thank you!