What are causes of bradycardia in the neonate?

Premature or normal. The heart rate is faster in the new born so that under 100 is usually considered to be bradycardia although that would be normal in an adult. Premature infants commonly have episodes of bradycardia and also apnea so that they not uncommnly end up on a home monitor/alarm. Rates below 100 are common and unless the rate is dropping below 60 it is unlikely a problem. Dr. Might recommend alarm though.
There're few causes. In premature infants the most common cause of bradycardia is apnea, secondary to underdeveloped respiratory brain center. In normal newborn it can be sleep-related. In pathological cases it can be due to cardiac rhythm abnormalities.