1)
Why did David compose this Mizmor?
1.
Radak #1: It is like the previous Mizmor. (One who was crushed in his illness should say it to praise Hashem after he is healed.)
2.
Radak #2: David said it over his illness.
3.
Malbim: It was when he was ill, and his enemies wanted to poison him, and he asked Hashem to save him. It tells Hashem's Hashgachah over the poor and weak.