Orthodox Christians could not conceive of simply sitting in the presence of the Lord especially while in worship. It is a sign of respect when a judge or even the President of the United States enters a room that those assembled rise up. Who comes invisibly unborn by unseen armies of angels. Those who are not able to stand for whatever reason are not, of course, expected to do so. You should do what you feel cojmfortable doing.
Many people get weary after a while.
So, they sit down. Other feel more comfotable remaining on their feet so as not to appear conspicuous. For Orthodox Christians, there must already exist the essential unity of faith between those who partake of the One Bread and the One Cup. Holy Communion is not a means to that unity, but is the fruit of a unity already reached. It is a tragic fact that Christianity is fontide into different groups, beliefs, and presuppositions.