trunk pops open about 1/4 inch in reverse

when in reverse the trunk pops open about 1/4 inch when the
backup assists start up. sometimes the retractable also comes out
but not always. Could it be the central locking pump located
under the back seat is confused? I disconnected the battery
trying to reboot the pump. No results. Also checked for leaks on
all vacuum lines at the pump. No leaks. Any help will be greatly
appreciated. 1993 300SD