What's the underlying causes of atrial fibrillation? echo done was normal.

Many causes.: There are many causes, such as ischemia, infection, trauma, neurological, metabolic and etc... You need to know what caused it.

Several: Look out for left atrial enlargement or mitral valve narrowing/leakiness on echocardiogram, which would indicate structural heart problems.. If echo is totally normal, rule out hyperthyroidism, sleep apnea, excessive alcohol intake.