I have an idea. The BR is loose because either…
- the motor / idler is too tight against the spool, causing too much friction and the belt is not retracting smoothly - or
- you could have an issue with the gear rubbing an unseated bearing (here: All belts retracting but one - #4 by gazinux)
(Is it possible the problem appeared when you put the new encoders on because you might’ve tightened the motor up too snug against the idler?)
You’ll have to take the arm apart either way 
Mine did a similar sort of drunken sailor swagger when I had the first issue above.