Like, how does the resonance happen before the cross over frequency? Can someone explain the intuition behind it?
Because of parasitics usually or from actual components. It's hard to say what it's coming from but the list includes the ESR of the inductor and capacitors or even compensation on the feedback loop. It occurs before the cross over frequency because any physical poles will create a pole that factors into the control loop.
In the case of a buck DC DC controller, it's the output filter that does (the paper shows the transfer function and the output response):