I also don't experience stress, and my resting pulse (50-54bpm) and blood pressure are very low. Interestingly, when I was a professional singer, under circumstances in front of larger audiences, I did experience fairly extreme stage fright, which was enough to impact my ability to sing at my best. Rapid pulse, "tunnel vision", forgetting lyrics, and bronchial spasms that prevented deep breaths. I now wonder if, especially the bronchial spasms, could have been due to the asthmatic symptoms brought on by the LPR...which, at the time, was beyond my understanding (I completely lost my singing voice a few weeks later).
As others have said, the causes of GERD can be many, and difficult to pin down. As my mother came down with reflux at almost the same time as me (although she's 27 years older), at least in my case, there appears to be some sort of predisposition.
Thanks for those great genes, Mom! ;-)