Safari sometimes opens a new window when I open a new tab and type something into the search field

I have set my Safari preference to "open new tabs". But, sometimes, when I search something in Safari in the Search field, a new window opens up. Only sometimes does a new window open up when I type in things like questions or any random thing. Most of the time a new tab opens up just fine though.

In other words, my problem is

When I open a new tab and I type something in the search field, sometimes a new window opens instead of a tab. Most of the time, a new tab opens up just fine, but I want to know why this happens