It's not a single chemical reaction- it's the epigentic expression of normally silenced DNA as the result of a hormone cascade. Sexual ambiguity in cannabis does seem to be most frequently triggered by photoperiod irregularity during the reproductive phase- It's not possible to state that the expression of ambiguity in an individual not known to be photostressed during flowering is the result of light stress earlier in development, as it's not possible to know that no other environmental stressor exerted influence in earlier development, because we don't know what's going to be a stressor for a certain individual. ( For instance, I cant eat FDA Red #5- blinding migraines.) We also cant know all of the possible stressors a plant has been exposed to, as anything in it's environment is a potential stressor, and we cannot be aware of all environmental factors.(For a sick joke some time, bring the ethylene level in your space up to about 150 ppm sometime- if you didn't know that the ethylene was there, you'd have no idea why all of your leaves were falling off. )

Reversal of sexual expression is a mechanism triggered by a reproductive strategy. For whatever reason, the plant has decided that it's best strategy to produce the maximum # of progeny that will survive to reproduce is to express both genders.
Lots of females will respond to non-pollination late in ripening phase by popping a few male flowers. It's pretty easy to see the reproductive advantage of this strategy- this plant will produce seed. Why not be generating pollen throughout the flowering cycle? While this stategy has a 100% chance of immediate success ( seed will be produced to reproduce next season), it's value decreases grately w/ each succeeding generation. If every individual were to only or to primarily self-pollinate, recessive negative traits will reinforce, leading to genetic drift ( which will limit the global total of individuals with which the individual can reproduce) and survival-negative genes will eventually accumulate, limiting the long-term viability of tthe genetic line. In the absence of another mechanism to limit the accumulation of negative traits, it's a dead end.

Most folks who have kept a male alive and flowering long enough will have seen a female flower on it eventually. This plant is saying to itself " Hey, I've been flowering for four months, so it must be like December! I'm still alive! This must be the best place on Earth! If I had kids here, they'd definately live long enough to reproduce!". The value of the potential breeding opportunities outweighs the cost of the reproductive investment ( forming female flowers instead of producing pollen).

Value={(potential of future reproductive opportunities) - (investment of reproductive resources)}. If Value>0, then the reproductive strategy is viable. Now the qustion becomes " What strategy will give me the highest Value?" . This computation is complicated by the presence of the two variables (potential/investment).

Now we get into the tendencies of gene lines, which can be likened to the personality of a chess player.
Some players are just more aggressive than others. Some will play a conservative game, and try to game the probabilities of each interactions. Others ( Bobbie Fischer?) will sometimes behave in seemingly random fashions which balance a low probability of return against a high value of return and a high cost.

High probability of return+high value of return+low investment of resource- you do it.
Mid prob of return+mid value of return+ low investment- you do it
Low prob of return+ mid value of return+mid investment- maybe you do, maybe not.
High prob+low return+mid investment- another maybe
Mid prob + low return+ high investment- most likely not

Furthur muddying the mix is the previous success of a strategy in a gene lines history. The individuals that pursue the most successful reproductive strategy in a given situation reproduce most successfully and therefore comprise a larger portion of the genepool that faces the next situation. If the same strategy is not the most successful in this new situation, the genepool of the next generation will be previously selected to pursue both stategies.

It's all a #'s game.

So what chemical reaction makes a plant go hermie? What makes a poker player bluff? What happened in Reykjavik? It's tough to know down to the molecule.

Certainly there are ways to chemically reverse sexual expression- colchicane will do it, as will ( anecdotaly) mega-doses of GA. But that's kinda like screwing up somebodies chess game by macing them- not whatchya'd call specific.

Hope this wasn't too long and obtuse for a non-answer.