Buck converters are prone to get unstable depending upon a lot of factors including variation in output impedance of the converter where the load varies from one value to another. The energy stored in the parasitic inductances of the PCB rings with the Coss of the MOSFET and can introduce high frequency ringing at the switching node. The higher the rise time of turn on of a switch, more will be the ringing. This ringing can make the loop unstable by reducing the phase margin. The instability of the loop at certain loads depends on a lot of factors like parasitic inductances, layout of the board, parasitics of the components and their tolerances etc... The main motive is to make the loop stable under all design specifications. That is why we always leave place holders for current limiting resistors at the gate of the MOSFETs to slow down their rising time if required.