What causes this??? I was trying to diagnose one issue and now this shows up.

It happens when I lock the BCG to the rear and slide the charging handle back into its locked position.

I have an upper that does the same thing.  The drag on mine appears to be the top nub on the front of the c/h riding against the top of the c/h trough.  The rifle functions fine except for the little bit of drag so I just live with it.  I have checked it with a RRA, BCM Gunfighter and a LMT c/h so I know it's the upper in my case.

Okay that's reasonable. I took out the hammer on my trigger last night and worked the action, and it didn't improve anything. Maybe, I'll just live with it. But knowing ME, I will probably end up having to order another charging handle from BCM.

I don't ever remembering this happening with any of my other charging handles on any of my other rifles. I'll do some swapping and some filing.

We had this problem before.  Thousands of chargers were made just a tiny bit out-of-spec, the lateral protrusions (the nubs that stick out on the sides, towards the chamber side of the charger were just a few thousandths too wide, this made it necessary to take them and just give them a light "tap" with a hammer.  You could try it yourself, we did it to thousands of them with no problems.  Lay the charger on it's side on a hard surface, preferably a vice, and then give the charging handle a firm tap with a hammer.  This will compress the aluminum where the ears/stubs stick out, but BEWARE, if you tap it too hard it will squish the channel where the gas key rides and then it will drag in that area, so go lightly and test, tap and test.  It has worked for us in the past.  You can also look for evidence of this problem by examining the charger and looking in the upper receiver where the nubs ride.  Good luck!