Lots of theorising here, without much thought about how to proceed with 'investigating'.
First, are you able to involve the state accident investigators? Often, they can be persuaded to examine a minor incident which seems unexplained and offers the opportunity to learn a good lesson before the same set of factors claim a life... I would have thought that if your licensing authority or insurer gets wind of your experience, they might want to look into it, and this would be a good way to do so.
Second, what about other aircraft which took off before or after you? Were there any? Have you spoken to the pilots? Were the ATCOs in the tower surprised at your prompt return to terra firma or have they seen aircraft in similar difficulty before, and if so, in what sort of meterological conditions? How about nearby airfields/gliding sites/whatever?
Third, and it is not normal to reach conclusions early on in an investigation, but have you considered talking to a local university meteorological professor or similar? It might be interesting to provide a set of met. charts, METARs and TAFs, which taken with knowledge of the topography, could lead to some thought as to whether significant standing waves might have been present. There may even be a facility somewhere that is able to 'model' the circumstances and give you some answers.
Fourth, and finally for the moment, what does your passenger recollect - is he/she a regular light aircraft passenger? Ask someone neutral to talk to him/her if you feel it might be of use.
Best of luck with resolving the question! Without professional help, it may be very hard, so I really would suggest pursuing my first suggestion.