From wikipedia article about Somatogravic Illusion
Vestibular/somatogravic illusions
Somatogravic illusions are caused by linear accelerations. These illusions involving the
utricle and the
saccule of the
vestibular system are most likely under conditions with unreliable or unavailable external visual references.
Inversion illusion
An abrupt change from climb to straight-and-level flight can stimulate the otolith organs enough to create the illusion of tumbling backwards, or inversion illusion.
The disoriented pilot may push the aircraft abruptly into a nose-low attitude, possibly intensifying this illusion.