回复：从美国移民加拿大申请美国签证被拒

Since your work in Canada and your background don't match and you worked in US within your background before, it is very possible for the VO to suspect that you are going to US for a job interview or job hunting instead of pure sight-seeing.
The VO may not think that a blue collar worker in Canada will look for jobs in US if the VO doesn't know that he has a Ph.D. degree.