There are several events, that have an effect on influence.
Two of them are random events, that either increase or decrease the influence of all races for a random amount of time.
Then there are two Mega Events. One dramatically increases the influence of a specific race, while the other increases the influence of all races, but one (it's in retaliation to "cultural conquest" by that race). They are permanent.
It sounds like you might have run into the second Mega Event.
You cannot disable individual Mega Events. It's all or nothing. While you can disable certain random events via modding, the influence-changing events, and some others, are hardcoded. So, once again, it's all or nothing.
During the map setup, remove the tick from Mega Events to disable them. To disable random events, set their frequency to None.