Thing is, and many adults don't realise/remember, easily done...................is that there are certain circumstances or places which kids really really don't want to be in. One place is supermarkets.
Everyone knows you get tantrum children in supermarkets, and it's not necessarily their fault, it's that they get unbelievably bored and start to react. It will depend on their personality, some will go almost into a mini depression, others will lash out.

Car journeys can be a problem, but not nearly as much as the former, kids do seem to adjust to these.

Now I don't know about plane journeys, but even for adults these can be really boring. Thankfully most decent carriers will provide movies and games, and it does help, as does looking forward to the slop they dare to call meals, as does the provision of free alcohol

Point to this is, maybe this is a reason for plane rage with very young kids.