Your juices are mostly all based on carbon chemistry (organic) in one form or another, and that includes flavorings, PG, VG, and alcohol.
If you leave something in a cooking pan too long, it will char. Foods, for example, are also based on carbon chemistry and consist of a large number of different compounds, and they start to break down the moment you expose them to heat, only some compounds break down at much lower temperatures than others. The difference between sugar and butter in a pan is an example.
The coil in an atty or carto does get up to around 500 Degrees which is enough to break down almost anything which you are likely to
vape. Organic chemistry is complex, and flavorings are complex compounds with complex molecules; so it becomes a mess to try to explain this any further.
My guess is that the culprit is the sugar, even if you don't think you have any. Artificial sweeteners are still various types of sugar.
It is about the briefest, non technical description that I can come up with.