You might have inadvertently set up an unhealthy environment in your must. Certain store bought cider, puree, and juice products are processed using potassium sorbate to extend their shelf life by inhibiting the growth of certain molds, yeast and bacteria. Always look at the ingredients to be sure that isn't the case with what you are planning to buy.
Also known as "wine stabilizer", potassium sorbate produces sorbic acid when added to wine. It serves two purposes. When active fermentation has ceased and the wine is racked for the final time after clearing, potassium sorbate will render any surviving yeast incapable of multiplying. Yeast living at that moment can continue fermenting any residual sugar into CO2 and alcohol, but when they die no new yeast will be present to cause future fermentation. When a wine is sweetened before bottling, potassium sorbate is used to prevent re-fermentation when used in conjunction with potassium metabisulfite(Campden tablets). It is primarily used with sweet wines, sparkling wines, and some hard ciders but may be added to table wines which exhibit difficulty in maintaining clarity after fining.
Some molds and yeasts are able to detoxify sorbates by decarboxylation, producing piperylene. The pentadiene manifests as a typical odor of kerosene or petroleum.