Specifically, how were the scablands of eastern Washington created?

Massive erosion was caused due to the Age floods acceleration across the tilted surface of the Palouse slope that resulted in the creation of channelled scablands.

Explanation:

Massive erosion was caused because of the Age floods acceleration that occurs across the Palouse slope's surface that resulted in the creation of channelled scablands. The eroded sediments were carried to the Pacific Ocean.

A huge lake known as Lake Missoula was formed during the Pleistocene Ice Ages and the lobes of ice that melt water from the glacier. Mostly the water over flowed the ice dam, forcing it to rise and break thereby creating a huge flood, that carved out both the channelled scablands as well as the Columbia River Gorge.