How might ammonia be created in this mechanochemical reaction? Can mechanical agitation catalyze reactions?  It appears so (thanks to Mithoron for the pointer!).

We observed the following: We were using 10-micron hBN powder to impact-plate copper bearings (e.g., by running them together in a vibratory tumbler, and other similar processes).  Even though the mechanical processes ran in a standard atmosphere and did not exceed 60C we consistently observed that ammonia was released.  As a control:  I have run the same copper with all sorts of other media through the same mechanical processes and never detected ammonia.

Our conclusion is that the BN is decomposing during the mechanical processing.  But hBN is very thermally and chemically stable: hBN doesn't react with Cu below 1000C!

So is it plausible that mechanical abrasion at this low a temperature would break the B-N bond?  If so, what subsequent reactions (again, in a standard atmosphere) might cause the free N to form $\ce{NH3}$ in detectable amounts, instead of just forming $\ce{N2}%edit$?
Fine hexagonal boron nitride powder is hygroscopic, so this implies there might be some water in your system. According to Reaction of Hexagonal Boron Nitride Nano-crystals under Mild
Hydrothermal Conditions "The reaction between water and hBN nano-crystals starts at very low pressure and temperature, i.e. 220 C and 1.0 MPa [~10 atmospheres]." 

Those conditions are likely to be met in a ball-mill or vibratory tumbler, at least momentarily at the points of impact (in a similar way, cavitation can heat water to luminescence!). 

A simple test would be to preheat the tumbler and its contents to 150 or 200 C for an hour or two, then seal it airtight before running the tumbling cycle. If there's no ammonia odor, then it indicates that adsorbed water in the hBN had been reacting with it, perhaps catalyzed by the copper bearings, as well as by impact heating and pressure.