Bobcat,
BNY Mellon is NOT the ultimate beneficial holder of the shares. They issue the Chorus ADR (Symbol: CHRYY), which enables trading in Chorus on the US markets. They hold the CNU shares as backing for the CHRYY they have issued (ratio 5 CNU to 1 CHRYY).
So the reduction in their holding means that quite a few holders of those ADRs (probably mostly US fund managers diversifying internationally into 'safe' utilities in 'safe' jurisdictions) have sold their CHRYY causing BNY Mellon to cancel those ADRs and selling the CNUs no longer required.
I doubt BNY Mellon makes any investment decisions here, they are mainly arbitrating and collecting fees for providing the service.
I would think their holding in MELCA will be for a similar arrangement, though I could not find a Meridian ADR.