Nothing prepared Egypt for the eclipse of royal power and poverty that came after Pepy II (Neferkare). He had ruled for more than 90 years (2246 - 2152 BC) as the fourth king of the 6th Dynasty of the Old Kingdom. Within the span of 20 years, fragmentary records indicate that no less than 18 kings and possibly one queen ascended the throne with nominal control over the country. This was the entire length of the 7th and 8th Dynasties (2150 - 2134 BC). In the last few years of the 6th Dynasty, the erosion of power of the centralized state was offset by that of provincial governors and officials who became hereditary holders of their posts and treated their regions as their own property.
By Fekri Hassan. It's a fairly pursuasive case for a severe drought right around the time of the end of the Old Kingdom. Climate change from around this time is known elsehwere in the middle east and as Hassan points out, from several other places around the world:
In Iceland, researchers have detected a transition from birch and grassland vegetation to arctic conditions in about 2150 BC. This correlates with a shift to drier climate in south-eastern Europe c.2200 - 2100 BC. Also, the reappearance of oak at White Moss, UK, suggests fluctuating wetness in around 2190 - 1891 BC. In Italy, drier conditions are found around 2200-1900 BC in Lake Castglione. Dry spells have also been detected as far away as Western Tibet at Lake Sumxi.
The most tantalizing recent discovery, however, was made when scientists made a high-resolution study of dust deposition from Kajemarum Oasis in north-eastern Nigeria. The study conclusively revealed that a pronounced shift in atmospheric circulation occurred in around 2150 BC. This data indicates that an abrupt, short-lived event of cold climate led to less rainfall and a reduction of water flow in a vast area extending from Tibet to Italy. This had catastrophic effects on such early state societies as the Egyptian Old Kingdom.
He only notes this at the end of the article (and tangentially elsewhere) that the nature of kingship changed from the OK to the Middle. Earlier, the king had been much more clearly divine with direct control over everything in Egypt. Later, his status had diminished somewhat with his primary duty as that of maintaining the balance and eventually gaining access to divinity in the afterlife. Being divine and having a horrendous drought ruin the country might tend to make one's followers a tad bit suspicious.
Hassan also deals with some of the hypotheses that have cropped up over the years related to the collapse of the Old Kingdom. For example, the idea that nobles had gradually increased their power and authority and asserted their power to the detriment of the ruling family. This has always been controversial and, as Hassan notes, there is little evidence of this supposed power outside of increases in tomb richness around Giza and elsewhere.
Good article and, as they say, read the whole thing.