This question is completely motivated by curiosity. It is a puzzle, not a problem.
The browser is Chrome. I have used it for a long time and sometimes do various things on it by rote. What I usually do for some applications that I use a lot is press the first letter of the URL and then press Enter (assuming I use it often enough that it comes up from the first letter alone) to go to that URL. As for this site, I press “B” and Enter and come to boards.straightdope etc. I do this in a new tab.
Sometimes instead of continuing in the new tab, I find myself in another instance of Chrome, another window, if you will. I generally don’t notice this for a while, sometimes not until I am about to close the tab, because the on-screen experience is the same up to that point. So I have no idea what keystroke or other action I have done to get a different result. In general, I don’t have more than a few tabs open at a time, usually only 2 or 3.
To repeat, this is a puzzle not a problem, and I am driven to ask only by curiosity not necessity.