Why do your feet shrink?

Yes, your feet can shrink. I do not understand how. I've seen others ask about their feet shrinking, so I know my foot shrinking is not unusual (if not common). My feet were a size ten when I was 18. Around 23, I was a 9-1/2. By 27, I was a 9. I'm 31 now and my feet are an 8-1/2. This crosses all manufacturers; Keen runs snug, and I'm an 8-1/2 in those. In sandals, 8's are comfortable. Usually I go down in sandal before I go down in shoe. Love my feet being smaller, but I wish I understood how/why.