Why did my dog act this way?

I'm not normally at home in the mornings, but today I was so I took my dog for a walk around 10:00. As we got towards my home the gardeners for the association areas had just arrived and were getting out of their truck. They did not have any machinery on or anything. As soon as my dog saw them (we were walking past them) she started to cower and act all submissive. She never does this. She loves people and always wants to go up to them to get pet and I usually have to pull her back so she doesn't bother people we pass on walks. I have one of those fences that is half wall and the top half is glass. When the gardeners are working she can see them and they often work right next to my fence. Do you think she associates them with loud noises and that is why she was afraid or do you think they may be teasing her or something? I just have never seen her act that way towards people before, dogs yes, but never people. The thing is I walk her a lot and we pass gardeners all the time, running blowers and lawn mowers. She has never showed any sign of fear passing them. It was just when we passed the gardner that works in my complex. It was just so weird. What do you think, why did she act this way?