Adam Clarke on Exodus 1:10

They join also unto our enemies - It has been conjectured that Pharaoh had probably his eye on the oppressions which Egypt had suffered under the shepherd-kings, who for a long series of years had, according to Manetho, governed the land with extreme cruelty. As the Israelites were of the same occupation, (viz., shepherds), the jealous, cruel king found it easy to attribute to them the same motives; taking it for granted that they were only waiting for a favorable opportunity to join the enemies of Egypt, and so overrun the whole land.