I think the crucial question here is where does "stroppiness" end and genuine "agressive behaviour" begin? Who will be the arbiter? Presumably the check-in staff, which in my opinion is not at all satisfactory.
Travelling by air is, for most people, a far more stressful occurrence than any other form of travel. People are usually travelling to a tight schedule, they know they have to get there two hours in advance but unforseen delays do happen, they may have a connection to make at the other end, they may have people waiting to collect them at their destination whom they cannot contact, they may be about to go on the holiday of a lifetime that they've spent years saving for, they may have a very important business meeting that is worth millions if it falls through, they are probably travelling to a foreign country and may be apprehensive about that. On top of all that, many people are not very comfortable with the whole concept of flying and are already nervous and on-edge. It is really hardly surprising that a lot of passengers are on a short fuse, and that they lose their cool when they get to the check-in desk and they find that they are delayed, or bumped because of over-booking, or denied travel because the flight is closed (after waiting in a queue for an hour). The icing on the cake is a smug or aloof check-in clerk who quite clearly doesn't give a toss that their whole world seems to be falling in around their ears.
This might well make them, by the standards of most normal conversations, appear somewhat rude to say the least. Surely this is not a reason to refuse to let them board? If they are using, or threatening to use violence or physical intimidation, sure. But rudeness??
To digress slightly. Has anyone else, apart from me, attributed any of the increase in air rage to the general practice of banning smoking in long-haul flights? Everyone is aware of the side-effect of nicotine withdrawal that makes addicts irritable. With something like 25-30% of the population regular smokers, isn't it likely that some of those are going to also find themselves on a short fuse some 8 hours into a smoke-free flight? I'm not saying that this is the exclusive reason for the increase in air rage, but surely it must be contributory?