inexplicable

/ˌɪn.ɪkˈsplɪk.ə.bəl/
that cannot be understood or explained SYN: incomprehensible

inexplicable behaviour
令人费解的行为
For some inexplicable reason he gave up a fantastic job.
由于某种莫名其妙的原因，他放弃了一份很不错的工作。