Seriously, why did Forza drop all of a sudden? It actually happened last week, but strangely it's only now people are seeing it.
Games that were already released for a while right now have no marketing and also didn't suffer a price cut + not included in bundles = no sales.
Dragon Age: Inquisition is the other perfect example. Recent release, no price cut on Black Friday sales and no bundles resulted in a major drop off in the charts. Far Cry 4 didn't drop as much because it was bundled in some unofficial XB1 bundles + official PS4 bundles and other retailers actually dropped the price on the game.