I have certainly never come across this nor head of it before. It would get me thinking to an underlying cause though. Does he have any other central or peripheral symptoms suggestive of anything else? e.g. neuropathy, paresthesia etc.
A heavy head could be the result of muscles relaxing that were once "holding". Dizziness could also result from an increased blood-flow as a result. Did you notice any cardiac changes during his symptoms such as an elevated or depressed pulse?
All things being equal and if there are no other signs or symptoms (and patient is willing) you might be able to repeat the treatment and see if it happens again. It may not happen the second time around. Give them a nice chair to relax in for 30 mins though. :smokin