Towards the end of my drinking...honestly just being awake was a trigger. Because if I was awake and not drinking it meant that I was in withdrawals to some degree, and the the longer I stayed awake without a drink the worse the withdrawals would get.
But before I reached that stage I would say fear was my biggest trigger. Fear came in many flavors for me and manifested itself in many different emotions...anxiety, anger, loneliness, resentment, jealously...you name it. But after sorting out my past it pretty much all boiled down to fear.
I was born with a heart defect and heart surgery has loomed over me my entire life (have had 2 of them in the last 14 years). I think that fear was ingrained in me at an early age, and when I discovered that alcohol eased that fear in my teens it was like the perfect set up for disaster.