Update: The negative feedback seems to have resonated with the Bing team. A spokeswoman just sent this statement (around 2:15 p.m. ET on May 9): "We heard the feedback, and we're disabling the feature now."
Sometime over the past couple of days, Microsoft began testing a new way to display Bing results. Those who are part of the test subset are seeing the results of their Bing.com search open in either a separate tab (as happens with Chrome) or an entirely new window (happening with Safari, according to some).
Microsoft officials said the change is just a test and that it is limited to the U.S. But Bing users I'm hearing from say the test is not limited to the U.S. and is happening in many other countries as well.