The MER aeroshell was different from Mars Pathfinder and Viking in that it had three limpet shapes attached to its backshell. These limpet shapes were the transverse impulse rocket system (TIRS). There is a very slight possibility that the TIRS caused this instability. Most engineers studying this MER instantly dismiss this theory because the pressure experienced on the backshell was very small compared to the forebody (the actual heatshield). It's hard to argue against this objection. However if the TIRS did not cause this instability then why didn't it occur on Mars Pathfinder and Viking 1 & 2? Another proposed theory was strong cross winds. However the Martian atmosphere has very little density and a wind could not have been blowing that hard. This is a real mystery.

Gary