Calling the question a riddle, I now realize , prompted people to submit answers. Thank you all!
The question: Rema (OC 51:7) states that there is a minhag to stand when saying VaYevarech Dovid.Yet no reason is given!
- Rav Yaakov Kaminetsky zt”l reportedly answers: The gaba’eiy tzedakah usually circulated around the Shul collecting Standing is an act of kovod for him, based on the Mishnah in Bikkurim that people stood up for the Jews who were bringing Bikkurim to the Beis HaMikdash.
- The MB there (19) says that the Ari z”l stood then and gave I assume he stood l’kovod the mitzvah he was doing. We follow that minhag.
- Another answer given is that this tefilah notes that Dovid HaMelech blessed HaShem and K’llal Yisrael. We stand to signify the chashivus of such a blessing.
- This from Dr David Blass: This is from Rav Yitzchak Lichtenstein shlita (Monsey, Rav Aharon’s son, Rav Yoseph Ber (JB) Solovetchik’s grandson), he may have said it in the name of R’ Chaim.
He said that the core definition of פסוקי דזמרה is saying of תהילים – as reflected in ברוך שאמר when we say ובשירי דוד עבדך נהללך , and according to the Rambam we do not say ויברך דוד through אז ישיר as they are not שירי דוד עבדך, and even according to us who do say it, we double the passuk כל הנשמה וגו׳ because this is truly the end of פסוקי דזמרה (language of that same רמ״א). So the three psukim of the paragraph ברוך ה׳ are the psukim at the completion of various ספרי תהילים, and according to this thesis they were put in davka there in the Siddur to reflect that this is really the end of the true פסוקי דזמרה …. And so just like we stand for ברוך שאמר and ישתבח as the ‘frame’ of פסוקי דזמרה, so too we stand for ברוך ה׳, and once we’re standing for that we stand until the end.
According to this, the ikar standing is for ברוך ה׳ and not for ויברך דוד.
- Rabbi Malinowitz suggests that since the passuk lecho HaShem hagedulah vehagevurah vetiferres….is the source for the the 7 middos of the sefiros (which are a total of 10: The 7 middos, with chochmah , binah , and da’as), we stand l’kovod that basic emanation of G-Dliness into this world.