Hyovolemia to extent that unable to compensate with elevated HR and increased peripheral tone to maintain BP when change from lying/sitting to standing
Neuropathy (e.g. from DM) resulting in impaired ability to raise HR or peripheral tone w/standing
Autonomic dz (e.g. multi-system atrophy/Shy-Drager, Parkinsons disease, Multiple sclerosis) that results in impaired ability to raise HR or peripheral tone w/standing; Vitamin C deficiency: impairs synthesis of catecholamines
Medications that blunt heart rate response (e.g. beta-blockers, clonidine) and/or decrease peripheral tone (e.g. alpha-blockers, calcium channel blockers)

Hypovolemia for any reason
Diabetes or other toxic/metabolic process leading to peripheral neuropathy
Multiple processes can contribute simultaneously (e.g. diabetic neuropathy + hypovolemia)
Meds that affect HR and BP

Lightheaded/dizzy sensation when move from lying/sitting to standing
More commonly causes presyncope rather than syncope
May have numbness, weakness, other symptoms associated w/neuropathy if that's the root cause

Orthostatic blood pressure and heart rate changes
Symptoms provoked with change in position
Impaired distal sensation, reflexes other findings associated w/neuropathy if that is the root cause