This behavior is actually expected and documented in AUCTeX manual:
5.4.1.2 Style Files for Different Languages
AUCTeX supports style files for several languages. Each style file may
modify AUCTeX to better support the language, and will run a language
specific hook that will allow you to for example change ispell
dictionary, or run code to change the keyboard remapping.
‘italian’
Runs style hook TeX-language-it-hook. Pressing " will
insert "< and "> depending on context.
If you're not satisfied with this behavior, customize TeX-quote-language-alist with M-x customize-variable RET TeX-quote-language-alist RET:
User Option: TeX-quote-language-alist
Used for overriding the default
language-specific quote insertion behavior. This is an alist where
each element is a list consisting of four items. The first item is the
name of the language in concern as a string. See the list of supported
languages above. The second item is the opening quotation mark. The
third item is the closing quotation mark. Opening and closing
quotation marks can be specified directly as strings or as functions
returning a string. The fourth item is a boolean controlling quote
insertion. It should be non-nil if if the special quotes should only
be used after inserting a literal " character first, i.e. on second
key press.
Using \usepackage[T1]{fontenc}, as suggested by @ChristianHupfer, is also recommended.
\usepackage[T1]{fontenc}, better use thecsquotespackage – Jan 17 '18 at 18:27csquotesand I usually use it. This time I was in a hurry, I'd loaded just a couple of packages and I didn't expect AUCTeX to do some magic even without it. – Arch Stanton Jan 18 '18 at 10:35