Perhaps the freezing of carbon dioxide presents an obstacle which stabilizes, at least for some time, the temperature at that value. As energy is removed from the atmosphere, instead of the temperature dropping the CO2 freezes. Does that sound plausible? And if this is the case, are other areas of the solar system similarly constrained to a temperature that produces a phase-change in some other atmospheric gas?
The mechanism is plausible, especially if you are measuring air temperature. However, that specific case is just a coincidence.
CO2 does freeze at -78.5 on Earth, but the much lower atmospheric presure on Mars moves the freezing point to below -120.