DOGs

I have a C5 Tourer and I am really very troubled that intermittently the tailgate opens all by itself when the vehicle is stationary. I carry three dogs in the vehicle and this has caused all sorts of problems with one dog being hit by another vehicle when the tailgate opened recently near a very busy road. We have disabled the remote control in case this was the source of the problem but the fault remains. Does anyone have any suggestions please? Regards Lynne for DOGs