I'm pretty sure I know why this happens.
I just went on a private server and spammed taunts while going through a teleporter and I eventually got it to work. http://puu.sh/gEiS1/8a05a77768.jpg
If you look closely, you will see that when you get teleported, your FOV starts higher than normal, and over a very short period, gets narrowed down to 90 (or whatever your FOV setting is).
So if you are able to taunt right as you get teleported, you will have the higher FOV, and for some reason taunting will keep the FOV from going back down to 90.
EDIT: I've just done some testing using host_timescale .1 (running the game at a tenth of the normal speed) and I've been unable to reproduce the bug even if I start taunting while the FOV is high, so it may be that it's just a random chance thing, or it may be that the timing is incredibly difficult. Either way, I'm very interested in this and I will keep testing.
EDIT 2: I spent another 30 minutes testing (I hope I wasn't supposed to get anything done today) and I can confirm that I was correct with my first paragraph, I could make a video about it to show it more clearly if anybody wants.