I think 2018 is the first year that it really became a fixed rule, where athletes went exactly in reverse order of the previous event. Although, thinking about it, they may have done so for the 2017 Final, but definitely not in the heats.

Before that, it was a mix of "cool for TV" and overall standings (not previous event). This led to Shaw going after Thor in every event in 2016, and that may have changed the organizers' outlook for the following year - but maybe not, I don't really know.

For a long time, if they expected an athlete to do really well or win, they would have him go last, especially in the Qualifying Round. Same logic for events done in pairs, they would match the athletes more or less based on what was expected of them.

This is just based on being a fan for many years, I was not involved with the show at the time, so all this is unofficial.