The crevice in the crown of the head is called ‘brahmarandhra’ since Brahman (Īśvara or God) is said to have entered this body through this randhra or crevice, after creating it, to make it alive (vide Aitareya Upaniṣad 1.3.12).
If a person manages to leave the body at the time of death, through this randhra, he goes to Brahmaloka or the world of Brahmā. Hence the name. Of course, this is possible only for great yogīs, persons of very high spiritual evolution.
Works on Haṭhayoga describe this brahmarandhra as the top end of the suṣumnā canal.