The entry for nope.avi doesn’t explain why the guy’s neck elongates. (Nor why his hat jumps up.) Is this a glitch from the game, a WAD aspect of the character, or something someone did in a level creator, or something else or what?
It’s not a glitch in the game, it’s something that was done “post production”, but I have no idea why.
This means a user did it. As for how, it’s part of the Source Filmmaker software:
There are programs out there where you can import items from Half-Life related games, mess around with the physics, use the game sounds (such as the nope) and record it. Garry’s Mod is a popular one, I believe.
Some videos, such as nope.avi, are short. There are other people out there who make more elaborate, awesomeones.