why does sysctl value change by itself?

In our firewall we use bridged interfaces. To call iptables for packets on bridged interface I set net.bridge.bridge-nf-call-iptables to 1

After a while, this value automatically set to 0 again. I am sure there is no cronjob or a configuration in sysctl.conf and sysctl.d contain this value.

OS is centos 7 and kernel version 3.15.9

What could be the reason ?