What’s causing this brick to deteriorate?

A more visible example of the type of discussion you’ll find at GBA. Here’s a portion of an excised chimney, suffering from efflorescence (the white brick is caused by deposits left by evaporated water), long-term neglect, and probably water infiltration of the chimney prior to its removal.

What’s causing this brick to deteriorate? | GreenBuildingAdvisor.com.