From Rabbi Pinchas Winston's commentary on parashat Vayeitzei:
According to the Radak (Tehillim, 91:1), Moshe composed this tehillah in honor of the tribe of Yissachar, who were constantly immersed in the joyous song of Torah. What is the joyous song of Torah? It is shirah, the song of the soul, for, just as the angels are constantly singing praises of G-d and His world, so, too, do our souls sing praises of G-d as well, all the time.
Also from his commentary on parashat Mattos:
According to the Talmud (Shavuos 15b), this psalm is called the "Song of the Plagues," because, one who says it and trusts in G-d -- one who takes refuge in the shadow of the A'lmighty -- will be saved in times of danger.
According to the Radak, Moshe Rabbeinu dedicated this tehillah to the Levi'im, who spent their days in the insulated environment of the Temple -- a true "fortress" of G-d. It was composed the day that the Mishkan was completed, and the Divine Glory descended to envelope it, into which Moshe entered to receive communication from G-d.
And his commentary on parashat Devarim:
"Eleven psalms were forgotten after Moshe's death, and these are the eleven Negative Forces of which there are eleven, the underlying basis of the 'eleven days journey.' The Midrash goes on to say that this corresponds to the eleven tribes that Moshe blessed before his death, in order to weaken these forces which are known to the Kabbalists as the underlying concept of the eleven curtains [in the Mishkan], the eleven spices in the Incense-Offering, the eleven verses that begin with the letter 'nun' and end with the letter 'nun' corresponding the eleven Negative Forces which cause forgetfulness ... which is why the eleven psalms were forgotten ..." (Yalkut Reuvaini in the name of Asarah Ma'ameros, Devarim 13)
Tehilim 90 - 100 are attributed to Moshe. Important to note, the shoresh לנן has two letters nun in it, yet it doesn't "begin" with a letter nun. It begins with the letter lamed ל, symbolizing "a heart that understands knowledge" (lev meivin da'at). Thus, the shoresh לנן is the tikun of one of these eleven negative forces.
It is fitting, therefore, that I employ Tehilim 91 instead of Tehilim 37 in my reversal and protection ritual.