As in all mythological stories, there is a story on how Maa Yashoda became the mother of Lord Krishna.
As per Bhagvata purana, Yashoda and Nanda in their previous birth, were a couple who made a great penance for Lord Vishnu. He appeared and asked them what they wanted.
They said with much respect, "Great Lord, we would like to treat you like our son." Lord Vishnu accepted. After all, he was never used to mother's love.
Then, in Krishna's Avatar, he grew up with Yashoda even though he was born to Devaki. In Bhagavatha we can see how much Yashoda loved Krishna.
Even when Krishna opened his mouth and showed her all the worlds, she could only see and stood astonished.
A while later, Krishna had to use his maya and let Yashoda forget Krishna's divinity. He wanted her pure motherly love to stay, instead of respect and devotion.
Though Mata Yashoda played an integral part in Shri Krishna's childhood, She once lamented to him that she did not get to witness his marriage.
This wish was fulfilled in the next birth when Krishna says that in the future he will take the Avatara of Sri Venkateshwara and then he will have Yashoda as his mother, Vakula Matha.
She could then, witness Lord Venkateshwara's marriages.
That way, Krishna became her child twice. Also, we know that in Tirupati, the most followed name of Lord Venkateshwara is 'Govinda', another famous name of Krishna.