“[I instinctively think that I would be mechuyav to save myself from danger, yet in my own case I am not convinced, although I cannot pinpoint the difference.]”
There’s a very big difference – there is no danger here! The only reason you are making the assumption that in that case, it would be mutar is because it’s to save your life, so you know that it must be permitted. That is not the case here.
There is also a second difference. Even in a situation in which your life is in danger, you are only allowed to protect yourself. You are not allowed to harm the person. By embarrasing the askan, you are harming him.
A third difference is that you are only allowed to be “choshesh”, but not to be “mekabel”. By protecting yourself, you are only being “choshesh”. In your situation, you weren’t just “choshesh”; you were “mekabel”.