This is because it is moved to a [+WH] COMP; the movement is hence wh-movement.
takahashi's (1993:660)  to the extent that the interpretive difference between (15a) and (19b) on the one hand, where reconstruction is possible, and (16a) and (19c) on the other hand, where reconstruction is impossible, can be captured by the distinction between wh-movement (for the latter) and scrambling (for the former), and inasmuch as the lack of reconstruction for long-distance displaced wh-phrases in cases like (16a) and (19c) can be assimilated to the scope property of the wh-moved phrase in English in (17a), the case for treating long distance displacement of whphrases in Japanese as overt wh-movement may seem prima facie strong.