This happens to me several times a day and I don't know why or what setting is causing it. It has something to do with tapping on the trackpad, but I can't even replicate it to figure out what combination of taps or fingers or swipes or whatthefuckever it is that is causing it in order to fix it. And there isn't a setting I can find that turns it off. With the trackpad, you can press down on a single word and it will pop up a dictionary definition of it. I love that feature and it's unrelated to either how I inadvertently open the dictionary application, and also to the application I'm talking about. I'm talking about the actual application, the opens in a new window as a single instance of the program, rather than a popup within another program. It's really driving me batty.