In every animation depicting the landing of the MERs, the tetrahedral airbag compartment is bouncing & tumbling after surface contact. How did Spirit & Oppy ensure they would come to rest upright? I feel like there has to be an obvious answer here that I am missing.
It turns out that there was. In this video produced for JPL (which I must have watched a thousand times as an excited kid), it is shown whichever of the three possible "not-bottom" sides/petals of the tetrahedral airbag & structure assembly it might have landed on, it would open first. In this way it would return to the proper orientation.
From the video, MER's aerobag assembly turning over.
This interpretation seems confirmed by this excerpt:
Twelve minutes after landing, motors will begin retracting the airbags, a process likely to take about an hour. Then the lander petals will open. No matter which of the four petals is on the bottom when the folded-up lander stops rolling, the petal-opening action will set all four face up, with the rover's base petal in the center. Opening of the petals is expected to take about 20 minutes if the spacecraft has rolled to a stop with its base petal down, about 35 minutes if one of the three side petals is down, or more than an hour if the rolling ended with the lander nose-down.
From a JPL press release, emphasis mine.