User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5043481-20130330104457/@comment-26074976-20130403203237

My idea is that the mould contains a mineral or chemical and the Brachy's saliva has an acid that reacts with the mould. When it punches this causes activation energy, which then speeds the reaction of the saliva and mould making it combust. The punches could just be a catalyst for the explosions to happen.