After running the following commands (bash) as root via SSH:
pkill --help pkill -h pkill /?
The first two commands didn't provide me any information, that's why ran the third (kinda instinctively...).
What happened next is that my SSH session to the server closed, and it wouldn't reconnect. I'm guessing that it stopped all (or most of) the running processes, including the daemon in-charge for such sessions.
I would like to understand why this happened: What is the exact evaluation (step-by-step) of my input and what did it cause.
My best guess is that it has something to do with the shell's evaluation of the '?' character, which probably translated to a list of a few single-character expressions which were passed on to pkill, which in turn closed these PID's.