Some of these are just strange with no explanation. Others, I think people are mistaking quotation marks for underlining, bold, or italics to emphasize words. Even an exclamation point would work better. There are even a blog and web sites on the topic, no joke, at http://www.unnecessaryquotes.com/ and http://www.juvalamu.com/qmarks/, among others. Quotation marks are used ordinarily to show quotations, or attributions in fiction to a particular speaker. They can also be used to offset story titles and other titles in part of a sentence. An increasing use is to use them to show irony, or sarcasm. Like you disapprove of your sister’s new boyfriend, so you write:
Yeah, he is a real “winner.”
In that case, to indicate the opposite, that he is a loser. So why would someone post a sign:
Employees must “wash their hands.”
It is not a quote, title, or attribution, so it can only mean irony or sarcasm. Are they supposed to wash something besides their hands? Are they not supposed to wash? Anyway, you get the point. Here is a gallery of them. “Enjoy.” [sic.]
Michael Bradley - Time Traveler 