Simply put, there are no tone sandhi rules for when words change from their citation tone to the neutral tone. The appearance of the neutral tone is morphologically motivated, not phonologically motivated — in other words, the tone change is not governed by the sounds of the surrounding words. Because the definition of sandhi is phonologically-motivated sound changes, the appearance of the neutral tone is not tone sandhi.
Certain words simply have the neutral tone because that is how their speakers have become used to saying them; for instance, many words that are pronounced with the neutral tone in Mandarin as spoken on the Mainland are not pronounced with the neutral tone in Mandarin as spoken in Taiwan. One example is 东西, which is typically pronounced as dōngxī in Taiwain rather than dōngxi. The phenomenon and its unpredictability is similar to the appearance of erhua (儿化) in Mandarin.