The triggers and the pre-thoughts work together. We already have the thoughts stored away in our minds and it then does not take much to trigger them off. I remember years ago even before I had a panic attack thinking "I am going to have a shit time at this party" and yes I had a shit time. I went into it with negative thoughts. My trigger/pre-thought was already there!!!
I still have the odd time of agoraphobia even though I do get out and about. I go to Edinburgh and back on a regular basis via train(its all of 50miles and takes less than an hour)..Most of the time I am fine but I can notice the trigger coming, the what if thoughts creeping in.
Its at this point we have to make a choice
A. Feed the thoughts
B. Change how we think