Why does my iphone weirdly autocorrect any in-app text field when I type out my email with '@@@'?
I would check your setting app first. 


General > Keyboard > Text Replacement


Using @@ to expand to your primary email and using @@@ for a secondary and so on is common practice. Perhaps you set that up, but in reverse. I believe I heard that from a podcast or some blog post a while back but cannot recall. I have adopted that shortcut and it’s super handy. 

Here is a blog crediting Rene Ritchie and the author, Patrick Welker, for the general awareness and specific use case of multiple @ to represent emails respectively. 


http://rocketink.net/2014/09/ios-@@-shortcut.html
http://m.imore.com/how-to-setup-text-keyboard-shortcuts-iphone-ipad-ios


Yours turning the actual mail into a shortcut, not so much useful.