Why does my debug session keeps sending requests to the server on its own?

I have a Windows 10 development machine with multiple web projects running locally using XAMPP. Here's how I set them up:

C:\Windows\System32\drivers\etc\hosts

127.0.0.1 www.site1.devel
127.0.0.1 www.site2.devel
...
127.0.0.1 www.sitex.devel

httpd.conf

<Directory "C:/Users/primordial_hamster/Documents/GitHub/">
    AllowOverride All
    Require all granted
</Directory>

<VirtualHost *:80>
    ServerName www.site1.devel
    DocumentRoot C:/Users/primordial_hamster/Documents/GitHub/site1
</VirtualHost>

<VirtualHost *:80>
    ServerName www.site2.devel
    DocumentRoot C:/Users/primordial_hamster/Documents/GitHub/site2
</VirtualHost>

...

<VirtualHost *:80>
    ServerName www.sitex.devel
    DocumentRoot C:/Users/primordial_hamster/Documents/GitHub/sitex
</VirtualHost>

sitex/.vscode/launch.json

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "PHP",
            "type": "php",
            "request": "launch",
            "port": 9000
        }
    ]
}

The problem: I run debug on a site that isn't site1 (first in config order). The debug session stops at a break point. I wait, and then suddenly vscode jumps to a line in the site1 project because it threw an exception. Looks like while the website was frozen, another request was sent (possibly the same one that I invoked on the website I was actually trying to debug), and was routed to site1 as some sort of default. This happens non-stop. The longer I wait the more requests I see popping up in the call stack. They seem to be about a minute apart. Why is this happening, and how do I stop it?