Baptists technically don’t have “denominations”. An individual can join the Catholic Church, the United Methodist Church, or TEC. But a Baptist individual really only “belongs” to his or her congregation. The congregation chooses to affiliate with the Southern Baptist Convention, one of the other Baptist conventions, or no convention. A convention is a free association of autonomous congregations that, for the moment, happen to be in fellowship.
A “convention” may produce Sunday School materials, endorse seminaries, or sponsor missionaries. Congregations can, and do, leave or join the Southern Baptist and other conventions, or go independent if they choose. I suspect secular society exerts indirect pressure on Baptist conventions to either adopt more liberal doctrinal positions - some have now endorsed legal abortion, and gay “marriage” - or simply deemphasize doctrine, as non denominationals do.
Either of these trends would tempt congregations to pull out of conventions that send out missionaries, since missionaries need a fixed doctrinal framework. If congregations are sliding towards liberal or lite doctrine, one way to save money is to pull out of SBC. There is no fixed boundary line here, a congregation can still advertise as Baptist even though it is essentially non denominational. The problem for Baptists is that the crucial element - freedom from man-made tradition and hierarchy - can be stretched much farther than the founders intended.
Let’s hope the SBC holds the line and lets congregations leave, rather than chase the trends.