As I reported above, the problem happened to me twice last week. I have NO extensions at all for Safari on my iMac. My Mom also has NO Safari extensions on her MacBook, and it happened to her, too.
I also have NO other user accounts, known or unknown, on my iMac, as suggested prior.
I, and quite a few others here, do not think it's a malware problem.
As I also reported above, I was able to trace the problem and isolate it to one banner ad (for Crate & Barrel) that showed up on the NY Times website, part of a rotating series of ads interspersed throughout the articles, down the page.
I also later recalled that I had the same problem many months ago with the San Francisco Chronicle's website. When I contacted their webmaster, he replied that it had to do with the ads and that they had no control over the situation, but that it would be corrected when the ad provider fixed the problem on their end. Just like these occurences, it was very temporary (going away by the next morning).
It's annoying as h•ll, but it's not a problem with anyone's computer. As smirking has said, if it is a problem, you can always use an ad blocker. (My Firefox had no ads, and I don't use Chrome, so I couldn't tell if the problem occurred with either of them.)