Ah, an area where I can share knowledge.
What happened was this: Huston has a law in place protecting the rights of sexual minorities. Over the summer, their was an effort to repeal it. Some pastors were subpoenaed because they were suspected of using their pulpit directly in the service of trying to get people to vote for the repeal, which I would say is a violation of the separation of church and state. Of course, since supposed persecution is the favored meme of conservative Christians now, they have made this into a huge issue to tarnish the name of Huston's mayor, who is a lesbian.