Soda and other carbonated beverages seem more "fizzy" when they are served on an airplane at standard cruising altitude. write a hypothesis that could explain this observation

the low air pressure reducing CO2 solubility

The differences between the airplane and standard cruising altitude are air pressure. When you are on the higher ground, the air pressure should be lower since there is less air above your head.
In a place with less air pressure, the gas solubility will decrease. Bubbles appeared if gas solubility is lower than the gas concentration. Assuming the beverage concentration is same, then it will be much more bubble appeared in an airplane because the gas solubility is lower.

That would be a working theory.