The same as many others - they were infiltrated by the relativists who began to reinterpret Scripture, rather than take it at face value. My mom has seen a real rollercoaster at our hometown church in the last ten or fifteen years. It's the typical battle of wills: religion influencing society vs. society influencing religion. It's like the programmers of MTV telling the keepers of the Word of God to rewrite the script. And IMHO, God gets a little pissed when you take His quotes out of context for your personal satisfaction.
What happened was that they have been overtaken by liberals, like most of the other main-line denominations. Going to a Methodist church in my area subjects you to contemporary music, skits, announcements for fundraising, and a sermon that reminds you of a talk at the Rotary Club.
Which is why I am becoming Catholic.