You raise a lot of good questions. I think the answers to at least some of this are already documented in the Nix Governance Constitution, which state that elections should occur in staggered terms, with the idea that on alternating years there would be either 4 seats or 3 seats up for election to a 2-year term.
In order to accommodate that through the first election, 4 members were (randomly? EDIT: see phaer’s reply) chosen to serve 2 year terms, and 3 to serve 1-year terms. Of the 4 SC members who were elected for 2-year terms, it appears that 2 have chosen to step down and the reasons will hopefully be made clear in their official announcements.
From my understanding, that means 5 seats (of the 7 on the SC) are up for re-election this cycle. There doesn’t appear to be a policy in place for how the terms may or may not be adjusted to attempt to re-create the initially envisioned scheme of alternating 4- and 3-seat elections.
I’m not sure what answer you are looking for on this one, as the SC was not determined by a single person with an available rationale to explain. I assume because the electorate found this desirable. The rules for determining eligible voters and for the election process are also documented on Discourse and github. The discourse link for this year’s SC election announcement and policy is The Election Committee announces the second Steering Committee election.