The only felony, if any, would be if an employee in the IRS released or leaked them to someone outside the agency.
These returns have "Client Copy" stamped on them. More than likely came from public records of a lawsuit Trump was involved in that required his return as evidence.
Or the best scenario would be that Trump pulled another of his famous head fakes on the media and released them himself.
Either way, another frickin' win for Trump!
According to the CFR publishing a private tax return by anybod is a 5 year felony.