What triggers Retrogrades?

In re: to Kathy's ? about what triggers a retrograde? Well I'm not exactly sure what you mean by that. But what comes to mind is that I think you might mean why some effect you as opposed to others? Let's just stick with Mercury for now. I think alot of Astrologers put too much fear into it & make too much of it. I'm not saying you shouldn't pay attention but sometimes due to circumstances you just can't put your life on hold & you have to proceed with business, but if you can wait till the cycle is over it has a better chance of working out. That's why electional charts are so hard to do. It's really hard to find that perfect chart. No matter how hard you try you can't take a planet from one part of the sky to another. Electional Astrology is where you try to plan an event to start at the right time. I think the degree the planet is going Retrograde has alot to do whether it hits your chart has alot to do with whether your going to feel it more than somenone else or if it's a strong planet or not. So for mercury, Gemini's & Virgo's might be more sensitive. I'm not really sure that touches on your question-let me know.