Ka is one of the names of Prajapati, a Vedic God. The reference to the deity is found in the famous Hiranyagarphasukta (Rig Veda X.121). Why Prajapati is known as Ka is mentioned in a story in the Aitareya Brahmana by Acharya Sayana.
Prajapati is known as Ka because of his unexplainable nature.
Some of the other explanations:
Ka is derived from root ‘kam’ which means to desire. According to this derivation Prajapati is called ‘Ka’ because he has a constant desire for creation.
Another derivation of Ka is from root ‘kam’ which means to make happy. Prajapati is said to give happiness to all creatures.