I have only done it twice... fairly recently, I replied to a couple ad. The pics were obviously fake, but I was told that they were "very similar" to what they looked like. As they were not "professionals" I could understand this but promised myself if they weren't pretty similar, I would bolt. When I was arrived, well, let's just say they were "similar" in the sense that they were actual people, but not only did they not resemble the pair in the ad, they weren't even close. She was really obese and he was not far behind. Just not my type at all. As I planned my escape, I finally decided honesty was the best approach and just told them I really preferred the body types in the photos, and that I couldn't stay. They were very gracious as I left. I resisted telling them that they had just wasted 2 hours of my time although I sent them a message later advising that "larger" people appeal to many people and that posting realistic photos would likely result in better calls and avoid awkward meetings like ours.
The other time was when I arrived at the location of a BP Indy. As I was heading to the shower, I heard someone moving around in the next room. I asked the girl who was there and she said "nobody". I closed the bathroom door... pondered, then just walked out. I heard her (or someone) open and then slam the door behind me and I half expected to turn around and see someone coming after me.
So I guess what causes me to bolt is dishonesty. If you are BBW, show it and let clients know up front. If your BF/pimp is going to be there, tell people up front. Maybe introduce him even if he has to be there.
To leave for a superficial reason like hair colour, or tatoos, seems a bit lame, although I think it would be best to disclose this upfront. I personally like ink on a lady, but not everyone does.