My opinion? Something caused it to finally get root rot. Rosemary is not known for generally doing well in a more tropical environment.

I'm in the subtropics, and believe me, in normal years we get incredible amounts of rain and humidity, and there's nothing wrong with my rosemary, lavender etc. It's the drainage that's important, more than the amount of moisture it gets. Rosemary simply will not tolerate having its feet constantly wet, and I strongly suspect that with the extra rain you've been having, on top of the soaker hose, you've finally managed to drown it. Take some cuttings and start again, because it's unlikely to survive once the root system has begun to rot from being soaked so much. You might just save it if you got rid of that soaker hose and allowed the soil to dry out again - lower down, I mean, not just on the surface. When you replant, mound up your soil to make a hill, and put your rosemary on top of the hill - that will give you better drainage.