All the electronics inside are very much capable of combustion.
Your power supply inside the printer body for example can very much fail and burst into flames.
And tbh it's not that uncommon for that to happen with 3d printers. They're often made with very cheap parts and prone to cheap work on the inside bits.
Add on how much of a high wattage load they meed to handle for extended periods of time and yeah, sometimes the inner wiring bursts into flames and the whole thing goes up.
I always recommend keeping a cheap lil smoke alarm directly overhead any 3d printer, seriously. Those fuckers can very much spontaneously burst into flames lol