From a medical stand point we don't know the cause of spontanious combustion, we just know it happens and sometimes people get hurt, then we rub some goop on them and they go home.
I see that I will have to be the one who does the diagnoses then.
Spontaneous combustion occurs when an entity has spent excessive amounts of time at an intersection of three or more ley-lines, causing an excessive buildup of magic energies due to the fact that living beings are
exceedingly good conductors of magic energies. These energies, however, then have nowhere to go if the entity leaves the cross of the ley-lines, and so will attempt to discharge to any nearby object that can accept the charge. Typically, this means that contact with another living thing, or a ley-line, will help to prevent the discharge of magic energies in more dangerous fashions.
However, there are a few examples where this discharge can result in significantly more dangerous results. For example, magical discharges into plants is known to cause fires well and often; it is for this reason that mages try to avoid going out much, as it would be dangerous for them to find themselves caught in a forest fire caused by their own stored magic. In addition, some people simply act as "magic sinks": regardless of their charge, they will continue soaking up magic energies until it discharges itself in a spectacular manner. Examples of such spectacular discharges include exceedingly improbable quantities of luck, in either direction (such throwing a knife and it killing 4 people before returning to your hand, or getting out of bed only to find that the entire fort is dead and your bed is suspended on a pillar above an enormous pool of magma), the random development of magical powers, the relatively benign "strange moods" that
dwa forumites are known to occasionally experience, and the aforementioned spontaneous combustion.
If you think that you can claim the "we don't know the cause of spontaneous combustion", then you need to go back to school and apologize to your magic theory teacher for not paying attention in class. We've known what causes spontaneous combustion for
decades.
EDIT: Just to be clear, the above is Erin.