Standing Up For The National Flag
It is only natural to have feelings of love for one’s homeland and respect for its flag and national anthem. Hence, there isn’t any problem in standing up out of these sentiments. You may also find the following video interesting: https://youtu.be/YCLEzFjCPWk
Is it shirk to make Sajdah in reverence for anyone other than Allah? If yes, then standing up which is a form of ibadah (as in Qiyaam) shall also be considered shirk.