Q. Why do some people sit and other stand when Kiddush is said?
A. There are different customs and no definitive law. In the Shulchan Aruch, the compiler, Joseph Karo, says that Vayechullu (Gen. 2:1-3) should be recited standing; the Rema (Moses Isserles) says it is better to sit (Orach Chayyim 271:10).
The argument for standing is that Vayechullu is our testimony that God created the world in six days and rested on the seventh. As witnesses stand whilst giving their evidence, so we should stand for at least this part of the Kiddush.