Complex sentence: The subprime mortgage crisis and high oil prices in 2008 resulting in the plummeting popularity of best-selling trucks and SUVs, perhaps forcing automakers to continue offering heavy incentives to help clear excess inventory.
Split into simpler sentences: The subprime mortgage crisis and high oil prices of 2008 caused the popularity of once best-selling trucks and SUVs to plummet. Automakers were forced to continue offering heavy incentives to help clear excess inventory.