What causes a spark plug to break off in the head
If tightening…too long a wrench, used by someone that cannot tell when a spark plug is tight enough.
I use a 3/8 inch ratchet and I grip it so my thumb never leaves the head of the ratchet.
As a friend always says.
Tight, tight…too tight broke.
If loosening, then the plug tas tightened too much last time.
But there can be a problem when using plugs that are not coated and installing them in an aluminum head.
This is not an old vehicle so the plugs should not need changing. How about some more information as to why you are asking and did someone break the plug.
Trying to remove it while the engine is hot.
The spark plug got bumped somewhere between the engine assembly plant and being installed in your car. Eventually it breaks off completely.