DAF HASHAVUA MEGILLAH 23b-Standing When Saying Elokeinu By Benching

ואין מזמנין (על המזון) בשם, פחות מעשרה 

There is Minhag , when Birchas Haziman is done with a Minyan, to stand up when saying “Elokeinu”.

Some are Machmir even when Elokeinu is not said to stand up at “Nevoreich Sheochalnu”.

 There is no Mokor (source) for this Minhag. We don’t find anywhere, someone who makes any Bracha,  is required to stand up when saying  the word “Elokeinu”.

The Aishel Avraham (Butshash)  writes that his Rebbe, R. Moishe Leib Sassover zt “l  stood up  during Birchas Hazimun.

He stood from beginning to end and not, just for the word ELOKEINU.

The reason for doing so, is, a Dovor Shebikdusha where 10 are required you are obligated to stand.*

The Chasam Sofer writes (Shu”t O.Ch. 50) When Chazal were Mesaken Birchas Hazimun, they specifically were Mesaken it, not to do so while standing. (argues on Reb Moishe Leib Sasover)

*(This is the reason why some stand during a Chupa)