The most common source is carried-forward losses. A company that lost money for years accumulates deductions it can use against future profit, and the accounting recognises that value today if management believes profits will arrive.
That belief is a judgement, recorded through a valuation allowance. Releasing the allowance creates a large one-off gain in net-income with no cash and no operating improvement, which is why effective-tax-rate can briefly go negative.
Example: Northwind Cloud carries $46M of deferred tax assets from pre-acquisition losses, against which a $12M valuation allowance is held because some expire before the unit is expected to be profitable.
Related: deferred-tax-liability, effective-tax-rate, income-tax-expense, earnings-quality, one-time-charge