Swept wing aircraft, when they stall at the tip, have a tendency to pitch up due to a combination of airflow over the tail changing and the C.G moving forwards. Other than this aircraft have a tendency to pitch down.
“The aeroplane may be considered stalled when the behaviour of the aeroplane gives to the pilot a clear and distinctive indication of an acceptable nature that the aeroplane is stalled”
One of these distinctive indications is a pitch down that cannot be immediately arrested and which is sometimes accompanied by a rolling motion that cannot be initially controlled.