TPS53667: pole appears at 4kHz on EVM

Hi,

I measured the loop of TPS53667EVM with 2 phases operation. It seems there is a pole or double-pole at about 4kHz, making the phase decrease from 180 to 40 degree(-140 degree). I would like to know where and how this pole comes? 

For the first figure, I injected 70mV through a 30ohm resistor in the loop, and set the BW limit as 1Hz,

For the second figure, it is 15mV and 1Hz.

Thanks.