P&R is a bit taken aback:
When did my church become a political lobbying group instead of a church?
And I’m not sure that I know the answer in detail, but I’ll take a stab at the answer in general.
The change occurred when the leadership in the church found that relevance to issues of current cultural awareness trumped the relevance of the life, death and resurrection of the Son of God.
In other terms, the leadership believes the past to be confusing (if not terribly misleading) and the future to be beyond understanding. That leaves them with only the present in which to seek justice.