Note the progression.
In the beginning, Pharaoh made his own choice to be stubborn:
"When Pharaoh saw that there was relief, however, he hardened his heart and would not listen to Moses and Aaron, just as the LORD had said."
"But Pharaoh hardened his heart this time as well, and he would not let the people go."
"When Pharaoh saw that the rain and hail and thunder had ceased, he sinned again and hardened his heart—he and his officials."
These next verses don't specify the agent but describe the state of his heart - passive statements:
"Still, Pharaoh’s heart was hardened, and he would not listen to them, just as the LORD had said."
"So Pharaoh’s heart was hardened, and he would not listen to Moses and Aaron, just as the LORD had said."
After Pharaoh demonstrates his own stubbornness, God is described as taking an active role in solidifying that defiance:
"The LORD instructed Moses, “When you go back to Egypt, see that you perform before Pharaoh all the wonders that I have put within your power. But I will harden his heart so that he will not let the people go."
"But I will harden Pharaoh’s heart, and though I will multiply My signs and wonders in the land of Egypt, Pharaoh will not listen to you."
"But the LORD hardened Pharaoh’s heart, and he would not listen to them, just as the LORD had said to Moses."
