On what factors they may have rejected my Citibank credit card application?
I have an account in Citibank with healthy balance and regular inflows. I have taken a few loans in recent times and did not default in payment. My credit score is around 560.
I also use my dad’s credit card (joint account) with huge credit limit and have excellent repayment history since I’ve been paying on this card for over 14 months. This would have been my own first credit card.
I’m in employment/business for last 3.5 years after completing college.

If you haven’t defaulted on your loans, your score should be higher than 560. You application might also have been rejected because you have too much credit. I recommend you request your free annual credit reports and see what’s on it.