Odd thing happened

Hadn’t used electronics in a couple of days while I was adding a place to hold laptop on side of worktable holding MPCNC. Started up repetier-host with intent to adjust positioning of x,y,z cable positioning along with DW660 power cable positioning.
When I attempted to move X axis all it did was vibrate violently over a distance of an inch or so. (It had worked fine the last time I had it all powered up.)
To see if it was the board I swap the X and Y leads. The problem stayed with the X axis hardware so that told me the board was fine.
I checked the tension on the rubber track and on the rollers and all seemed to be fine.
On just a whim I swapped the polarity on the mid connection around (on the wiring kit) knowing it should have no effect and it didn’t. BUT, when I put the connection back like it was originally the X axis worked just fine!
Anyone have any idea why?

Given the symptoms and what you did to ‘correct’ it, I’d guess at a wiring short/break in your harness for that axis. I built a Delta printer a couple of years ago and I hand wired and soldered all of the cabling for the motors and had a similar sounding issue with the extruder motor. It would shake and shudder, sometime going to opposite way. No seeming rhyme or reason. Upon closer inspection I found a bad wire in one of my sleeved connections. Fixed and it was fine. Try moving the axis over a long distance while fiddling with the wiring to see if you can make it misbehave. If so, you found your culprit.