What is the single most common driver of a deferred tax LIABILITY, and walk through why the timing works that way.

Accelerated tax depreciation (MACRS / bonus depreciation) versus straight-line book depreciation on the same PP&E. In the early years tax depreciation > book depreciation, so taxable income < book pre-tax income; cash taxes paid are LOWER than book tax expense, and that excess book tax expense accrues as a DTL. The asset's tax basis falls faster than its book basis, so book basis > tax basis = DTL. It reverses in later years when book depreciation exceeds tax depreciation, drawing the DTL back down. Over the asset's full life total depreciation is identical — it's purely a timing difference.