Why does the sentence "Sidiĝu sur ĉi tiun seĝon!" require the accusative ending, but "La familio sidiĝis ĉe la tablo." does not? They both refer to movement into a sitting position in a specific place.
Great question. I've written in detail elsewhere and won't repeat myself - but in short -- early speakers didn't use an -n on sentences like "Sidiĝu sur ĉi tiun seĝon!" -- and I think they had logic on their side.
I think the difference is between whether the action was on-going or was completed. To me was seated is the result of sidiĝis rather than the action itself; i.e., became seated describes the action of sitting down, while was seated described the action after sitting down.