Okay, an answer.
It is a matter of causation. What is directly causing the social stress the target takes? Is it magic directly or is it an event that is carried out via magic.
Take your example of dropping someone's pants. In this case, it is the act of the pants dropping in front of others that is causing the social stress regardless as to the exact mechanism by which one uses to bring the pants down. The use of magic is just a means that could easily be replaced by normal actions for the same result. In this case, after the pants are down, the individual who is now embarrassed can still attempt to play it off (roll rapport perhaps?) and avoid the stress. Or, someone can step in and use the moment of the pants falling to lay into the poor guy to add to the stress inflicted. In both cases, the magic is used as the method by the which the pants are dropped and it is the targets inability to adjust/play it off that causes the stress with the lack of pants being a hindrance to him. Which is exactly why this is magic being used as a maneuver to a social attack; either to add a +2 to someone's attack or as a compel.
Take the other example of using Air magic to talk to a person such that no one else could hear your words. Easy enough to accomplish with magic (though it would be debatable as an evocation). But, really, what is causing the social stress in this instance? Is it the magic, or that the person is reacting irrationally in front of others? It is the later. After all, if you used the same magic to secretly communicate with an ally, your ally would not take social stresses from the conversation. The Air magic is a means. It is the carefully chosen words spoken (Empathy Roll perhaps?) designed to evoke a reaction from the target in front of others. In this case, magic would be a maneuver as well that places an aspect along the lines of "Voices in your Head".
Even the mass Entropy Curse example is simply a giant maneuver (or maneuvers) that takes a toll on a person. But it does not directly attack his social standing.
This is because of the nebulous nature of one's Social Stress Boxes. This represents, one can argue, the combination of a characters social standing in society (whatever that society at that moment happens to be) and his own self-image. In essence, it is about perception. His perception of himself as well as others perception of him. And, yes, I'm using a generic "him" pronoun here... could be a her or it. To attack this, you have to alter perceptions. You can do this, as is most often, though manipulation. Or, if you have magic, you can use it to directly alter one's mind and adjust that perception. BUT, if one goes the direct route, you are certainly breaking a Law of Magic there. You can still do it... but you are breaking the Law by altering someone's mind.
The indirect method, though, is to simple create a situation where people perceive something that causes them to change their own mind. That can be seeing a guy lose his pants; for sure. But it is the pants dropping that changes their perception and, therefore, inflicts the social stress upon the target... not the magic that caused the pants to come down.
For magic to be the direct cause of social stress means that magic would have to directly inflict stress upon one's social standing.