Some ideas:
Some triggers are environmental. Your "porn time" comes after dinner, before you watch TV at night. That time of the evening is a trigger. Maybe an evening beer goes with it. That can be a trigger.
Some triggers are emotional. You might think about an ex, an attractive woman at work, family problems, other insecurities, and it ups your cravings. This was true for me in a major way. It was only when I realized on a deep level that porn was making it worse. We get it in our heads, but it takes a long time to understand it deep down and break that connection.
Some triggers are deeply emotional and psychological. I struggled (still struggle, honestly) with an acute sense of loneliness. Sundays were hard... I'd go to Church, see the families happily spending time together, I'd go home to my lonely apartment and for months I was good at getting through every day except Sundays.
Sometimes the triggers are sheer curiosity. My favorite porn sites of choice were message boards and cam sites. Those two are absolutely poisonous because they potentially could have something amazing at any minute that I'm NOT looking right at it. The idea that I could be missing something incredible (spoiler... I never was) would haunt me.
Sometimes triggers have triggers, which have their own triggers. They exist in such a weird, long train that doesn't make much sense. Think about Pavlov's dogs. The experiment is over applied in pop-psychology, but the basic phenomenon is this: the bell led to food, which led to salivation. Take out the middle thing, and they still salivate. A bell causing salivation is a complete non-sequitir, but the association was very well trained in their brains. In the same way, you may have had a long train of thoughts or behaviors that led up to porn, and because porn hits the brain so hard, it can create new associations that don't seem to make much sense. Just the basic acts of your evening routine can trigger porn because of those associations.
What can help is to retrace your steps and figure out when you went into "zombie mode". At some point you'll notice that you just seem to go through motions towards porn, and it feels almost like you're watching yourself on a screen do those things and can't control it. If you try to modify your behaviors at those points, you'll frustrate yourself. If you retrace your steps back far enough where you were either just starting to go into zombie mode or before you entered, then you can find behaviors you can modify. Often they'll seem rather trivial, but that's okay. That's how these addictions work. Maybe you'll have to give up something else, find a different place to be at certain times, or just give yourself well timed pep talks.
