Paradoxical laughter is an exaggerated expression of humour which is unwarranted by external events. It may be uncontrollable laughter which may be recognised as inappropriate by the person involved. It is associated with altered mental states or mental illness, such as mania, hypomania or schizophrenia, and can have other causes.
Paradoxical laughter is indicative of an unstable mood, often caused by the pseudobulbar affect, which can quickly change to anger and back again, on minor external cues.
This type of laughter can also occur at times when the fight-or-flight response may otherwise be evoked.
References
Paradoxical laughter Wikipedia(Text) CC BY-SA