Every time I do open a markdown file in VS Code, it opens up in Chrome as well (in preview mode). When it does it also brings it to the top and it's annoying closing it each time. I don't want it to do it, also because VSC already has a function to show the preview of .md files so it's useless opening a Chrome tab.
The problem was caused by the dbankier.vscode-instant-markdown extension.
In general, if you're experiencing weird behaviour, you can check if it's caused by an extension by doing the following: Does it happen when you reload VS Code with extensions disabled (use the Developer: Reload With Extensions Disabled command in the command palette)? If not, then do an extension bisect to figure out what extension is causing it.