my dentist told me it's due to a loose ligament. the ligament probably got loosen when you yawn or laugh or open your mouth too wide. yup, it's not a big problem according to my dentist. quite a lot of people have this problem so don't worry too much! =)
but don't keep straining the loose ligament deliberately or else it may be dislocated. lols.
i believe it's the Temporomandibular Joint Disorder...
one way to aid "recovery" is to leave your jaw neutral, position your tongue to rest on the upper row of teeth whereby it's like when you're saying the letter N, remember to not flex your jaw muscles, this aids the recovery of the joint itself.