Safari randomly opens current tab into a new window

[dorsy28]

I have a MacBook Air running Safari 9.0.1. Lately, when working in a tab, safari will randomly open the current tab into a new window. I thought maybe it was a mouse setting, and even purchased a different mouse. Sometimes I won't have even clicked anything, just moved the mouse on the pad. Other times I will click on something in the tabbed site, and it will appear in a new window. When I go back to the original tabs, the page is not there. It is like it was REPLACED into the new window. I do a lot of research, and have multiple tabs open at the same time. When this happens, I have to copy/paste the window URL, close it, and re-tab it.

Any ideas? This has made my research cumbersome.

Thanks.!