http://www.tourismthailand.org/Thailand/weather
I have never been in Thailand but it looks like the rainy season lasts from may to november. The monsoon coming from the southwest brings humidity&rain.
I guess it makes sense that if the males are flowering and you have a nice rain/high humidity (like seen in Vietnam war films) the pollen of the males is not going to work.
The Land races there adapts to the situation and triggers some male flowers, in order to ensure the SURVIVAL.
We know that some african lines like Durban, are hermie prone too.But in Africa the humidity/rain can´t be the enviromental factor. I think the cause could be draught/very dry years may be?? I can´t think of another cause.
Not trying to stir the shit but again I think there is a big difference between male flowers produced by a female plant NATURALLY in response to a enviromental situation VS male flowers produced CHEMICALLY by the human race in response to a GREEDY MENTALITY.
My 2 cents
Thailand has many areas with different climates. it is mostly hot there, and there is a long rainy season--but in the north, where cannabis is cultivated--it is pretty dry most of the year. so probably dry enough for pollen to spread without being deactivated by water.