Malbim: If you rely on people, you rely on Mitzrayim, like Malchei Yisrael relied on it (Perakim 30, 31); it is like a fragile reed.

Rashi, Radak: A broken reed, when he leans on it, the [pointy] exuding chips enter his hand [and puncture it].

Rashi (Melachim II, 18:21): When one leans on a fragile reed, and it breaks in his hand, the [pointy] exuding strands [where it broke] puncture his hand. So Pharaoh does not help those who rely on him. He only harms! Malbim - there are two losses through relying on a fragile reed for support. (a) He does not support himself with his legs ¹ . (b) The reed breaks, and punctures his hand. So those who rely on Pharaoh, they slacken from acting to save themselves, and he comes to collect tax from them.