Peaceful times in the east came to an end, as expansionist attitudes came over the rulers in Asia.
Asshur decided he had to "go forth" out of the land of Shinar, and make himself a habitation further north. That must have pressed painfully upon other groups of people in the region.
An aggressive spirit came over the Elamitic kings, and they began military expeditions starting at the shores of the Persian Gult and ending in Southern Syria and Palestine.
The migrations of tribes with Terah and Abraham from Ur to Haran, and from Haran to Hebron, is one of the indications of the restlessness of the period.
The Hittites were growing in power, and required a larger territory for free expansion. Such expansions would force a large population from Upper Syria to migrate southward.
There are signs of pressure on the southern border of Egypt from Asian immigrants looking for a home, as early as the beginning of the 12th dynasty. Asiatics were from time to time allowed to immigrate to Egypt during the reign of Amenemhat I. The eastern delta was more or less Asiaticized. And a large portion of its inhabitants were inclined to welcome more Asiatic immigrants.
So these expansionist moves by Asian kings were changing the landscape of the entire Egypt. And eventually they reached Egypt.
The following account comes from a supposedly native Egyptian historian Manetho:
"Certain men of ignoble race, coming from the eastern regions, had the courage to invade the country, and falling upon it unawares, conquered it easily without a battle. After the submission of the princes, they conducted themselves in a most barbarous fashion towards the whole of the inhabitants, slaying some, and reducing to slavery the wives and the children of the others. Moreover they savagely set the cities on fire, and demolished the temples of the gods. At last, they took one of their number called Salatis, and made him king over them."
Salatis made Memphis a city to put his throne in, collected tribute from both Lower and Upper Egypt, and went to great lengths to fortify his new kingdom from a possible attack from powerful Assyrians.
If Manetho is to be believed, there were around a quarter million invaders.
It could be debated who they were. They are called Hyksos. They could have been a mix of different groups, similar to barbarians who conquered Rome. Modern Egyptology tends to believe they came from a Semitic Canaanite background. Also, while some historians believe the invasion was massive and fast, others believe it crept in gradually.