The Supreme Court sought to examine the crime of DUI under Florida law, and how it applied to immigration law. Under Florida law, DUI has no mens rea element. That is, it does not require the defendant to have a level of intent to commit the crime. The court determined that DUI does not in fact entail a substantial risk that physical force will be used, making it a non-violent crime.
As a result, the Court decided that Leocal's crime was not means for a deportation order, and he was let free. The decision was reinforced by the fact that Congress explicitly distinguished crimes of violence and DUI crimes. The negligent nature of a DUI crime is different from a crime of violence, because it does not require proof of a mental state.