Finally, transport studies are done, which demonstrate stability for limited periods of time outside of normal conditions eg high temperature excursions if sat on an airport in the sunshine in Texas for that bottle of pills, or perhaps a few hours at room temperature in this case. These can include freeze/thaw cycles as that can affect product performance more than temperature change per se.
Now some speculation: This vaccine has not had very long in development at all, so stability data and understanding will be much more limited than usual. There may be almost none for some manufacturing sites. Also, it might be very difficult to characterise degradation for a long RNA strand - I doubt it is possible to precisely characterise, and I've no idea what criteria might be used. It might be that -80 is used because of the difficulty of defining how much degradation is acceptable, or to standardise between sites of manufacture.