Question: Why do euphoria boners happen?

I have a question: Why do some trans women experience euphoria boners from gender euphoria even though they don't want to? Do AMAB people in general experience sexual arousal as being linked to certain emotions. Like when you're sad you get aroused, when you're happy you get aroused etc. I know that there is the theory of misattribution of arousal where fear gets confused for romantic arousal in both men and women, is something similar going on here? if so, why is it more common in trans women than trans men? I personally am of the belief that euphoria boners are not related to fetishism of being a woman etc as some people suggest, but I am curious to know what the actual cause might be?

I don't see a lot written about it online, and I know that not all trans women experience it, but a certain portion definitely do, and it causes them distress because they otherwise have no erotic attachment or perception to their transition or euphoria.