Personal answer:
I don't know exactly what caused my autonomic dysfunction. I know that my illness started out as primarily Neurally Mediated Hypotension (pre-syncope symptoms were my biggest problem, the increased heart rate was not as pronounced). I got an NMH diagnosis after a tilt table test in 1995. My heart rate increase was not quite high enough to be called POTS. Now, after 20+ years of illness, I seem to have a combination of both NMH and POTS (after testing at home and by my cardiologist).
Unlike some ME/CFS patients who only developed autonomic problems after several years, I had NMH symptoms from the very beginning of my illness. The dizziness, feeling short of breath, and nausea (probably other symptoms, too, but these are the ones that come to mind) started a month or so before I got too sick to work and had to take medical leave.
I do know that my illness followed a severe virus. Correlation does not mean causation but it's likely that this virus is what triggered my illness. But anything more than that is pure guesswork.
General answer:
There are a lot of factors which might contributed to POTS. I don't think doctors know all the factors that might contribute, nor can they always diagnosis which factor(s) applies to each patient. But in case it helps, here is one list of possibilities from the Dysautonomia International web site:
http://www.dysautonomiainternational.org/page.php?ID=150
A brief extract (bolding done by me):
As you may know, dysautonomia is not a specific medical diagnosis. Dysautonomia is an umbrella term used to describe any malfunction of the autonomic nervous system. There are many underlying diseases and conditions that can lead to dysfunction of the autonomic nervous system. This is not an all inclusive list, so check with your doctor if you believe you might have an underlying cause for your POTS or other form of dysautonomia. Remember that not every dysautonomia patient can find a specific underlying cause, and some of these conditions are very rare.
I hope this helps! But just be aware that you might not find out exactly what is causing your POTS.
Even without knowing the cause for your POTS (or other autonomic dysfunction / orthostatic intolerance) it's still worth trying to figure out how to treat it. Knowing the cause can often help the patient choose more effective treatments, of course. But not everyone can figure out that underlying cause.