I believe that humans do not have the ability to just randomly burst into flames, and instead cases of Spontaneous Combustion are caused by the wick effec, which I will get into in a second. However, I do believe there are cases where investigators are unable to find an ignition source for burn victims.
Also, The human body is made up of 60-70% water, so it would be very difficult for someone to catch on fire without an external source.