The SW_ pins have a low series resistance in the path to accommodate the need for high cell balancing currents.This coupled with the internal ESD structure will allow for a high current path internal to the part from the SW_ pin to the HV pin during initial hot plug in which the HV cap is not charged.The addition of the diode is to shunt this initial surge current external to the device.We recommend this diode being able to handle a surge of 3A for 33us.DCIN and C12 have higher resistance in series which will limit the current for these pins and thus no external protection is requiredThis is determined by the application circuit (please refer to the EV Kit for exact detail). The DCIN pin will have 100 ohm series resistance, the Cx pins will have >1.5k series resistance while the SWx pins only have 12 ohms (this is limited by the desired cell balancing current).
Without this diode, the current will flow into part's internal ESD structures from SWx to HV.