Dirty Hands for Devious Reasons
As with virtually all Fetishes, the reason(s) why someone has a Sexual desire for Dirty Hands could be virtually anything. Yes, many Fetishes have a common causation, but there always are (and always will be) exceptions to the proverbial rule.
One man, back in Queen Victoria's England of the late 1800's, was one whom admittedly housed a Sexual Fetish for Dirty Hands; having a quite healthy 'hard-on' for the working women of the age.
But we don't have to speculate or take some so-called Educated Guess as to why he had such a Sexual Partialism. For, you see, he was quite kind to leave us his diary in his wake −one which he meant to be read (so you need not feel guilty... for when your're invited to a Threesome, you can't say you are a Peeping-Tom).
He had quite eloquently explained the workings of his sexuality.
(I can't help but wonder if he was conscious of the comical connection of his act of splattering ink with the flick of the wrist upon the telling of his lust for dirty hands.)
This man's Hand-Fetishism was actually based around the Strength (willpower/spirit, that-is) of working women in his day. He explained that the thing that sparked his psychosexual enjoyment was behind the dirt upon a woman's hands; and no, he wasn't speaking of her skin beneath. Rather, he was attracted to a woman's determination to continue; her ambitious acts to obtain & gain.
In other words, behind the soot on a woman's palms, he saw her passion for profit, her strength in making sure that a Shilling (money) was coming her way.
(In this sense, we can say that he was also into the Sexual Paraphilia of Financial Domination.)