The shift seemed to happen around 20 years ago. My late uncle was a Nebraska State Patrolman, and later a cop in Omaha. I knew most of those guys on his shift, and while there were some a holes, they were very good guys.
When the next generation of cops started (my cousins and others I grew up with), they changed focus. For starters, many have told me they would just follow orders, no matter what those orders would be. My uncle did not and would not do that. The other shift was the guys I knew all of my life began to seem themselves as above the ordinary people, or as equivalent to soldiers in Iraq (as in fighting a war). Never mind the Army and Marine cousins and friends of mine do not like that attitude. (In general, the regular guys HATED reserve forces made of from police officers. They got to many people killed).
My uncle viewed his job as serving the public. My cousins view it as controlling the public (their words, not mine). My uncle was a hard a$$, but he always treated people with respect. My cousins have been known to pull their side arms over traffic violations (and they are not alone in that).
When police start acting like occupation forces, and start seeing themselves as such, don’t be surprised with the regular people start treating them as such. I used to be VERY pro cop. I would give them the benefit of the doubt in general. Now, because of those I know, I am no longer that way. Times have changed, and not in a good way.
I too have an Uncle am extremely proud of. He worked his way up from the Patrolman position to become Chief of Detectives. Loved Him! I will admit to also having seen a bit of change in my area but I am from the Hornet’s Nest. Was Born and Raised in the Hornet’s Nest. Am proud of LE in my neck of the woods. They do a difficult job and the majority of LE in this neck of the wood are still Great Guys. The neck of the woods I live in has been fortunate. I pray the same for the rest of this nation. May God Make It So.
Look at how the rhetoric changed in that era, though. It went from fighting crime to the "war on this" and the "war on that", as if that justified the extraConstitutional tweaks in tactics and shift in equipment.
When everything is a "War", you're going to get a Fort Apache attitude.
Movies were as responsible for the shift as the daily media rhetoric, but protect and serve went out the window in a lot of places on account of the 'war' and the 'us vs them' attitude (easy enough to get when you deal with dregs daily), and it became CYA and get home, put in 20 or 25 and collect your check.
I'm not saying all LEOs are that way, not by a long shot, but there are enclaves where the attitude seems to prevail.