It looks like the Bureau of Labor Statistics was forecasting growth ~20% for NMT as late as 2012 (See Salary and Employment Outlook at http://learn.org/directory/category/Health_Professions_and_Medical_Services/Health_Diagnostic_and_Treatment_Services/Nuclear_Medicine_Technology.html).
I suspect what happened is people rushed into this fairly small field and saturated it ~5 years ago.