Here are a few possible laws and traditions that could create such a situation (some more credible than others).
Guilty by lack of evidence
A law was passed to deter people from doing false accusations.
If the plaintiff can't prove the crime did happen, he is systematically charged and convicted for said crime. If the judge asks for too many pieces of evidence (multiple witnesses, culprit's confession, weapon's receipt, etc...), a trial can quickly turn against the plaintiff.
This situation can result from a general culture of mistrust, where the public is prone to doubt everyone's word.
Blame the victim
This can take multiple forms (see Tim B and King-Ink answers):
- Blaming people for not protecting themselves enough, for attracting criminals or for being at the wrong place at the wrong time
- Accusing victims of crimes to bring shame to their families, their kingdom or their race
- etc. etc.
These laws can exist in a society that despises weakness.
The one who smelt it dealt it
Following an ancient tradition, if the Dwarf Police don't find any suspect during the year following a crime, the person who reported said crime becomes the prime suspect.
If your dwarf society is structured by a rigid caste system, it could be a crime for a commoner to accuse someone above them in the social hierarchy.
(This is close to the "Blame the victim" solution)
Blame the boss
In a society with strong family values, the head of a family could be held responsible for the actions of all the other members of said family.
In this situation, if someone steals from their own family, their grand-mother, uncle or father could be prosecuted with them, even if they are the victim.
The same logic can apply to clans, guilds, companies, etc.
Someone has to pay
There is nothing the dwarf government dislike more than losing money. If the defendant can't pay the bill, then the plaintiff has to.
For example, if a homeless dwarf A deface the property of dwarf B, is found guilty but can't pay the fine, then dwarf B has to pay it.
The dwarf justice system never doubt the content of an official form.
If the cop who took the deposition mixed up the plaintiff and the defendant's names, the plaintiff will be prosecuted.
You can mix some of these solutions together, and maybe add a bit of xenophobia and incompetence to it.