Why does bradycardia happen?

Bradycardia. Bradycardia, in adults (not in children) is defined as heart rate slower than 60 beats per minutes. It can be perfectly normal in people who are athletic. For example, marathon runners can have resting heart rate in 40s or even lower. It is, generally, pathological, if it leads to symptoms of dizziness, lightheadedness or fainting. If it causes extreme symptoms, a pacemaker is needed.
Bradycardia. Bradycardia (slow heart rate) has many causes ranging from vagus nerve stimulation (sticking something at the back your throat) in a normal infant to various types of heart block where there are problems with electricity system of the heart. Atheletes tend to have bradycardia which is normal. If you or someone has bradycardia that causes symptoms (dizziness, fainting) see a cardiologist.