adorabilis I can't definitively answer your question, as it's entirely muscle memory, but I can give you an explanation as to what I believe is the cause of it. It essentially originates from the action of wanting to change the page you are on.
In every context in which you want to change pages (ie. visit a different site), you press cmd+L. So the action of changing the page in your second tab would be ctrl+tab (alternatively, cmd+2) followed immediately by cmd+L. Thus, the action of going to a different page is directly associated with the cmd+L to focus the address bar.
The only context in which cmd+L is not needed is when opening a new tab (and by extension a new window). However this is a highly infrequent occurrence since every other context requires focusing the address bar. As such, the muscle memory from pressing cmd+L in every other context "bleeds through" so to speak.
Alternatively, in Chrome, pressing cmd + L after opening a new tab shows recent queries (or recently visited sites in the case of Firefox).