My thought is this is not a new and brilliant idea hatched out when the Fed refused to fund pet projects.
The sanctuary cities are some of the wealthiest areas in the country, either in personal wealth or large, multi-national companies. These entities need cheap workers that are too afraid to report any malicious activities, including paying wages that are far lower than the law allows.
Houston is doing the same thing. Original Fed request was $331,000,000. Congress said no, you should be able to pay for that with the 8.25% county sales tax. Now the amount is in the billions.