I had heard an explanation from Late Dr. Ramanand Sagar and one from ISKCON. According to one, Kubja was Manthara in her previous life and as per the other opinion, she was Surpanakha. Because of her bad deeds during Ram avatar, she was born with that hunch back or trivakra apprearance in her next life . But as Lord is ever merciful, He in His Krishna Avatar rescued her.
But as I have written before, I haven't read it anywhere, only heard about it. If I manage to gather some information from ISKCON ,can post it later.
In case you are interested, would like to share few other characters;
1. More or less, like this, Putana also had her fate. When Bhagwan Vishnu in His "Baman Avatar" went to Daityaraj Bali, one of the ladies there seeing Lord's innocent face had a desire in her heart to get Him as her son. But as soon as the Lord asked for "teen Paag Zameen", that time she also said in her mind, if she was His mother, she would have given him poison. That exactly what Putana did, she tried to feed poison with her milk to Bal Krishna.
2.Similarly the "dhobi" bacause of whom Lord Ram had to abandon Mata Sita was killed by Shri Krishna on his way to Mathura palace.
3. And also, Lord Rama cleard all his debt to Sugreev's brother Bali, if any, when Bali was reborn as Jara in later yug, and his arrow actually shot Lord Krishna's lotus feet and became the cause of His death.