When did David compose this Mizmor?
Radak: Also this was when he fled from Avshalom.
Malbim (1): This was when there was a famine for three years. Scoffers said that it was due to David's sin. David asked Hashem; He said that it was "El Sha'ul v'El Beis ha'Damim Al Asher Hemis Es ha'Giv'onim" (Shmuel II, 21:1). David appeased the Giv'onim, and rain fell; he then offered "Zichei Tzedek" (verse 6), i.e. Todos, that Hashem clarified that it was not due to David's sin. This Mizmor is divided into three; each part ends with "Selah".
Why does it say "Elokei Tzidki"?
Radak: You know that I am justified [in my feud with Avshalom]. The injustice is with my opposition. You, Elokim, will judge us. 'Elokim' is always a judge - "Yesh Elokim Shoftim ba'Aretz" (58:12). This also applies to human judges - "Ad ha'Elohim Yavo Devar Sheneihem", "v'Higisho Adonav El ha'Elohim" (Shemos 22:8, 21:6).
What is the meaning of "ba'Tzar Hirchavta Li"?
Rashi: [I was in difficulty] in past days; now Hashem graced me and heard my Tefilah.
Radak: You will release me from constriction. In Nevu'ah, past tense is often used in place of future; for it is as if the matter was done already.
Malbim: [A second reason why You will answer my Tefilah is] due to mercy. You always answer in times of affliction, and release me from constriction.
Why did he say "Chaneni u'Shma Tefilasi"?
Malbim: This is the third reason why You will answer my Tefilah - amidst a free gift - "v'Chanonti Es Asher Achon v'Richamti Es Asher Arachem" (Shemos 33:19).
