Why did David compose this Mizmor?
Radak: It is for everyone to say when he is crushed in his illness or affliction of his soul.
Malbim: It is when David was mortally ill, and recovered. It was after the episode of Bas Sheva. His enemies rejoiced over his illness and had thoughts about him.
Why does it say "Lehazkir"?
Rashi: To say it at a time of Tzarah to mention Yisrael's afflictions in front of Hashem. It was said for all of Yisrael.
Radak #1: Perhaps it is one of the musical notes.
Radak #2: It is like to thank. This is like "Hazkiru Ki Nisgav Shemo" (Yeshayah 12:4).
Radak #3: It is to inform every downtrodden person to say this prayer (Mizmor).
Malbim: It is to remember his sins, and they will always be in front of him.
