I've never done a subject-to, but I have read a little about it. From what I can gather, it all centers around fraud. Fraud can happen from either party: buyer or seller. Fraud is more than a civil violation, I will be a criminal one if a prosecutor decides to take the case.
It looks like most of the allegations of fraud come in the form of failure to fully disclose or to falsely disclose. In other words, fraud by omission or fraud by commission. Maybe someone lies about their credit worthiness, income, existing liens, etc.