there is an opinion that standing/sitting relates to who you are addressing: Hashem or congregation. Thus shome esre standing, shma sitting.
there are other considerations:
– standing near an open window might be stealing fresh air from others (Rav Salanter)
– sitting in a place you are not a regular of, might be taking a makom kavua from someone. (AAQ). But I have some support: I came to a place that I visit sometimes and wanted to take my usual standing place. (according to above). To my “annoyance”,, the standing place was already taken – by a chassidishe tzaddik from further away place. He was apparently holding by both my shita on not taking makom kavua and also by coming early.