Witchdoctor , good points.
However it’s not always that clear cut.
The passengers can just come across in good spirit initially.
Check-in agents are trained to warn passengers who turn up at check-in under slight influence of alcohol that they could be denied travel if they do not stop.
For the majority of the population this does work and a note can be entered into the system to say they were warned.
The passengers may have become worse after boarding the aircraft. I have seen passenger’s act sensible until they are passed the gate staff, because they know they are at risk of being offloaded.
The other more unfortunate reason could be the shear time factor. Unfortunately driving force in most turn-arounds. The fact the passengers were slightly happy may have been missed at that stage.
All just suggestions from my own experience.