Unless the judge was striking down the federal immigration law (which he wasn't) he over stepped his authority.
The judge was neither striking down federal immigration law, nor overstepping his authority.
From
https://www.gpo.gov/fdsys/granule/USCOURTS-azd-2_07-cv-02513/USCOURTS-azd-2_07-cv-02513-21
"FINDINGS OF FACT AND CONCLUSIONS OF LAW - that Plaintiffs are entitled to injunctive relief necessary to remedy the Fourth and Fourteenth Amendment violations caused by MCSO's past and continuing operations. The MCSO is thus permanently enjoined from: 1. Detaining, holding or arresting Latino occupants of vehicles in Maricopa County based on a reasonable belief, without more, that such persons are in the country without authorization. 2. Following or enforcing its LEAR policy against any Latino occupant of a vehicle in Maricopa County. 3. Using race or Latino ancestry as a factor in determining to stop any vehicle in Maricopa County with a Latino occupant. 4. Using race or Latino ancestry as a factor in making law enforcement decisions with respect to whether any Latino occupant of a vehicle in Maricopa County may be in the country without authorization. 5. Detaining Latino occupants of vehicles stopped for traffic violations for a period longer than reasonably necessary to resolve the traffic violation in the absence of reasonable suspicion that any of them have committed or are committing a violation of federal or state criminal law. 6. Detaining, holding or arresting Latino occupants of a vehicle in Maricopa County for violations of the Arizona Human Smuggling Act without a reasonable basis for believing that, under all the circumstances, the necessary elements of the crime are present. 7. Detaining, arresting or holding persons based on a reasonable suspicion that they are conspiring with their employer to violate the Arizona Employer Sanctions Act. IT IS FURTHER ORDERED setting a hearing at which the above matters will be discussed for Friday, June 14, 2013 at 9:30 a.m. in Courtroom 602, Sandra Day O'Connor U.S. Federal Courthouse, 401 W. Washington St., Phoenix, Arizona 85003-2151. Signed by Judge G Murray Snow on 5/24/2013. (KMG)"
Arpaio detained people for potential immigration issues based on their apparent racial heritage, without other probable cause, in violation of the constitution.
He was ordered to stop doing that.
He didn't. Hence his conviction for contempt of court.
The original order didn't stop him enforcing immigration law, it simply said that an apparent Latino heritage was not probable cause for an arrest.