In Tanna Devei Eliyahu Rabbah (Chapter 8) it is explained that most of the plagues came as a response to the Egyptians' schemes to diminish the Israelites. This is the text:

**Blood**: Why did it come upon them? Because they withheld water from the Israelites to prevent them from immersing and multiplying. Therefore, the Holy One, Blessed be He, turned their water into blood, as it says (Psalms 78:44), "He turned their rivers to blood." An Egyptian would say to an Israelite, "Give me water." He would give him water, but it would turn into blood. The Egyptian would say, "You drink it." The Israelite would drink, and it would be water. The Egyptian would say, "Let's drink together from the same vessel," and they would share it, but it would be water for the Israelite and blood for the Egyptian.

**Frogs**: Why did they come upon them? They told the Israelites: "Go and bring us reptiles and insects, and we will play with them as much as we want." Therefore, the Holy One, Blessed be He, brought frogs upon them until their voices were heard from the bellies of the Egyptians, saying "ko lekho." One scripture says (Exodus 8:2): "The frog came up." And another scripture says (Exodus 7:28): "The river shall swarm with frogs." How were these two scriptures fulfilled? Rabbi Akiva says, there was one frog, and the Egyptians hit it with sticks, and frogs would spring out from it until the whole land of Egypt was filled with frogs.

**Lice**: Why did they come upon them? Because they made the Israelites sweep houses, fields, roads, and streets. The man would sweep the woman's house, and the woman would sweep the man's house. Therefore, the Holy One, Blessed be He, turned the dust of Egypt into lice until the Israelites could not find any dust to sweep.

**Wild Beasts**: Why did they come upon them? They told the Israelites: "Go and bring us wolves and lions, and bring them into our warhouses, and we will hunt them as much as we want, so they remain in the outer deserts and do not enter our homes and multiply." Therefore, the Holy One, Blessed be He, brought upon them all the wild beasts in the world until they confessed their sins.

**Pestilence**: Why did it come upon them? Because they made the Israelites shepherd horses, donkeys, camels, cattle, and sheep so that they would be in the deserts and not enter their homes and multiply. Therefore, the Holy One, Blessed be He, brought a pestilence upon them until the Israelites could not find any livestock to shepherd.

**Boils**: Why did they come upon them? Because they made the Israelites heat their hot water and cool their cold water. Therefore, the Holy One, Blessed be He, brought boils upon them so they could not touch water, whether hot or cold. What did the Israelites do at that time? They would go and bathe in the water and enter their homes with joy.

**Hail**: Why did it come upon them? Because they made the Israelites plant gardens, orchards, and all kinds of trees, so they would be in the outer deserts and not enter their homes and multiply. Therefore, the Holy One, Blessed be He, brought hail and broke all the plants the Israelites had planted.

**Locusts**: Why did they come upon them? Because they made the Israelites sow wheat, barley, and all kinds of legumes so they would be in the outer deserts and not enter their homes and multiply. Therefore, the Holy One, Blessed be He, brought locusts and ate everything the Israelites had sown.

**Darkness**: Why did it come upon them? Blessed be the place, Blessed be He, who does not show favoritism. There were sinners in Israel at that time. Darkness was for the Egyptians and light for the Israelites so that the sinners in Israel could be buried without the Egyptians saying, "Just as there was a plague upon us, there was a plague upon them."

**Why did their firstborns die?** Because it says (Exodus 1:16), "When you help the Hebrew women in childbirth... if it is a son, kill him." Consider it as if it had already been done. From here they said, the nations of the world are seized for their thoughts as if they had acted. But Israel is not seized until they actually perform the deed.