While it is certainly possible you got sick on the airplane itself, don't factor out the trip to the airport (cab/public transit) and all the different crowds your have to deal with at the airport itself. I know airport staff do the best they can, but during particularly busy travel times airport bathrooms can be incredibly disgusting in my experience.

Sample size of many, but correlations doesn't imply causation.

Evidence for limited spread on airplanes is pretty solid, but everything from here on out is speculatory:

* A lot of people get sick traveling. You're interacting with all sorts of people from far away who have novel viruses, many of which aren't circulating in your area. That doesn't mean you catch it literally on the airplane.

* Masks probably make a bigger difference on airplanes than elsewhere. Even cloth masks catch big particles. By the time droplets evaporate into small droplets, odds are they've passed through a HEPA filter many times.

Rhinoviruses spread not only via aerosols, but also supposedly spread better through surface contact than COVID19. Air circulation won't stop you from touching a contaminated surface.

But its pretty clear medicine has a very poor understanding about how viruses spread given how long they denied coronaviruses were airborne. So who knows.

If travel is stressful, or causes you to get less sleep, (etc) that may also affect your immune system.

In addition to the other responses, plane cabins have very low humidity, which can take out your mucosal protections from getting a cold. It's really common to at least have a scratchy throat after flying just from dryness, and often times catch a cold afterward.