Honestly I don't know what triggers it most of the time for me; I think my brain is mis-wired due to pressure on my brain when I was an infant. I had spinal meningitis when I was a few months old and they did tests on me fort he next 15 years to try and figure out what the full impact of that was. I was diagnosed with GAD when I was 5 and had my first panic attack when I was 4, so it's pretty much just always been there, although sometimes it can seem to go away for months and then other times it seems like it wants to control my life.
There are some things that I know can set me off; pretty much any physical symptom that has caused it in the past or any new weird symptom. There are some symptoms that I've had for so long that they don't set me off anymore; pretty much anything gastric for instance, since I've had IBS for so long it's just normal to me.