When the Kansa decided to perform Dhanush Yajna to increase his age and to kill enemies, he invited Nand along with his sons. For this, he sent Akroor ji to Gokul. He asked the wrestlers to kill Krishna & Balrama on their arrival. Shri Krishna and Balrama came along with Akroorji.
Stories of Shri Krishna in Mathura:
When Muchukund saw Shri Krishna, he requested him for undistracted Bhakti (devotion). Shri Krishna said, “Muchukund! You cannot get undistracted devotion in this life; in youth, you were extremely lustful or libidinous and luxurious”. Hence, he has to take one more birth. In next birth, he would get birth in Brahmin family and would get undistracted devotion. Muchukund did hard penance and born as Narsingh Mehta in Kalyug.
Thereafter, Shri Krishna preferred to leave Mathura in the interest of Mathura habitants. He left for Okha, Gujarat. BalKrishna stayed up to the age of eleven years in Gokul and thereafter up to the age of twenty five years in Mathura.
Shrimad Bhagavad Mahapurana.