First Mining Gold Corp (FFMGF)

[Implanting]

It makes no sense to me why they are changing the ETF by rebalancing the holdings within it the way they are.

The ETF is supposed to be composed of "small-cap" mining companies and they are reducing those companies and adding bigger cap companies. That changes the high beta returns the smaller miners normally bring to the investor. That's the exposure you desire when investing in an ETF like the GDXJ in the first place.