SB 10.4.14

tayābhihitam ākarṇya
 kaṁsaḥ parama-vismitaḥ
devakīṁ vasudevaṁ ca
 vimucya praśrito ’bravīt
tayā — by the goddess Durgā; abhihitam — the words spoken; ākarṇya — by hearing; kaṁsaḥ — Kaṁsa; parama-vismitaḥ — was struck with wonder; devakīm — unto Devakī; vasudevam ca — and Vasudeva; vimucya — releasing immediately; praśritaḥ — with great humility; abravīt — spoke as follows.
After hearing the words of the goddess Durgā, Kaṁsa was struck with wonder. Thus he approached his sister Devakī and brother-in-law Vasudeva, released them immediately from their shackles, and very humbly spoke as follows.

Kaṁsa was astonished because the goddess Durgā had become the daughter of Devakī. Since Devakī was a human being, how could the goddess Durgā become her daughter? This was one cause of his astonishment. Also, how is it that the eighth child of Devakī was a female? This also astonished him. Asuras are generally devotees of mother Durgā, Śakti, or of demigods, especially Lord Śiva. The appearance of Durgā in her original eight-armed feature, holding various weapons, immediately changed Kaṁsa’s mind about Devakī’s being an ordinary human. Devakī must have had some transcendental qualities; otherwise why would the goddess Durgā have taken birth from her womb? Under the circumstances, Kaṁsa, struck with wonder, wanted to compensate for his atrocities against his sister Devakī.