I have a thought, maybe we're looking at this from a more short term view then should be.
What being a cop is considered for some people a secure job they can attain when other more desirable options are not available. I mean you always need cops and you often need them more when the economy is down.
When the economy goes up there more desirable jobs more available so people take careers in those fields instead of going into law enforcement.
I'd find it hard to believe that a movement that just started getting coverage a year ago would make a bunch of people suddenly decide they don't want to be cops anymore. It's possible but I just tend to see more at play.