The story is full of twist and turns. I will give the information i know. It is very interesting.
To know more about the story, you should know the god Tvastar. Tvastar is referred as the god who created sky and the earth, created different creatures. He created different objects like the soma cup that was used to drink soma (a vedic ritual drink), created vajra (a weapon) for Indra. He also created an axe for Brihaspati. In the Rig Veda, he is shown as an artist who creates a lot of different and unique things and that's why he is also known as the heavenly builder.
But something that was not created by Tvastar is water. Water was already present. I know things have started becoming confusing. This is what i know.
Sky and the earth were created by Tvastr to be his house. Later sky and earth became parent of lord Indra. Indra told his mother about the way he wants to get borned but the mother earth opposes his request. This is where the hymn 18 comes into play.
THIS is the ancient and accepted pathway by which all Gods have come into existence. Hereby could one be born though waxen mighty. Let him not, otherwise, destroy his Mother.
Mother earth for some reason wanted to abandon Indra. So, mother earth left Indra but Indra kept following her. While following his mother, Indra reached the place of Tvastar.
Tvastar is the guardian of soma. Now, Indra had to drink soma to become powerful. Some say that Indra stole soma from Tvastar. After drinking soma, Lord Indra became powerful and as a result it was getting hard for mother Earth to hold him.
She asks Kusaava ( a river) to take him in her womb. Later the water of Kusava helped Indra to grow. Lord Indra kept on growing. As, I already said Indra wanted to be delivered in an unusual way. He wanted to come from side and he was continuously growing. You can look for Kusava in the Rig veda.
This made difficult for Kusava to hold him. This is where the other lines come in
He bent his eye upon the dying Mother: My word I now withdraw. That way I follow. In Tvaṣṭar's dwelling India drank the Soma, a hundredworth of juice pressed from the mortar.
Kusava could not hold Indra and as a result Kusava dies.
This story is really interesting. I am not 100% clear about one thing and that is if Indra took soma before getting in the womb of kusava or after.
Anyway, as I said the story is full of twists. As, I said Tvastar was the creator of lot of thing,
If we look at Tvastar then it's him who wanted to dethrone Indra. To dethrone Indra he created Trisiras (Tvastar three headed son).
Tvastar in revenge for his son created Vritra. Vritra was the one who holded all the water and Indra had to kill Vritra to release water.