Most kung fu systems come with an origins legend. Most involve a monk or a peasant who had some life-experience, went into the forest and observed something, and came out 20 years later with a new system that enabled them to defeat all the existing masters and local thugs until everyone pledged loyalty to them. By comparison, the stories of Wing Chun's origin are somewhat believable, but there is still a reasonable question as to whether they are entirely accurate.
There are a few explanations for why kung fu systems may have created these legends. One is simply old-school, folk marketing. Another may well be related to the term “Crouching Tiger, Hidden Dragon”. Prior to the days of YouTube, masters and sifu tended to avoid calling attention to themselves. People would, and still do in some parts of the world, come to a village and ask “Are you the master of this system that I've heard about?” They would reply, “Oh no, that's not me, I make tofu...or repair shoes…or...” To get him or her to reveal themselves took time and trust. They didn't want to invite challenges and they didn't want the dynasty or the neighboring village they were fighting with to burn their house down. Instead, they sent them off looking for a monk or a nun or a farmer who didn't exist.
I don't know the degree to which the Wing Chun legend is true and accurate. I will admit to being a bit of a skeptic, but here are a few versions of it below, both taken from my SiFu's site. There are a number of other sources and variations of this story on-line and in publications as well and it is a good read.