I gave a lot of thought to triggers when I had my first really bad stuttering patch in years shortly after I finished high school. (For context, I went through high school with several of my teachers not realising I had a stutter, then began a time period where I didn't want to go out socially anymore because I couldn't hold a conversation.) I found that the more people there are in a conversation, the worse it was. People naturally interject when someone else might not be done speaking, the subject changes quickly, etc. So there's a lot of pressure to a) finish what you're saying before someone interjects and b) jump in with what you want to say before the subject changes. I do find the same thing one-on-one sometimes, too (though it is much easier to circle back after a subject change; but I digress). None of these issues are present when you're giving a speech. So while you might not actually be talking more slowly, your brain is able to focus better on the actual speaking rather than juggling the mechanics of conversation. That's my theory, anyway!