LTC4366-2 Problem

I have designed surge protector with LTC4366.

I attached the schematic.

My problem is, when load current is 10A there is no problem. But i increased the load current to 15A LTC4366 stopped working and voltage across mosfets became near 2.5V.

Voltage between out and gate pins is -0.0024V.

What is the reason for LTC4366 i can not understand. I observed same problem with LTC4356.

Edit : LTC4366 is not damaged. Only Q2 damaged. When i change Q2 mosfet it worked.

But what is the reason for mosfet damage? Should it be C9 capacitor? Or should i attached snubber to mosfets?

Edit note.
[edited by: hguney at 9:19 PM (GMT -4) on 29 Sep 2018]