LyX allows the user to tailor the input-experience through what is called "key bindings". Informally they are sometimes referred to as (key) shortcuts. An excerpt from
Customization.lyx, section 3.3. Bindings:
Bindings are used to, well, bind a function to a key. Several prepackaged binding files are available: a CUA set of bindings (familiar as the typical set of PC and CDE set of keyboard shortcuts), an Emacs set of bindings, for those of us who follow the One True Way and refuse to lower our standards, I'm kidding here, of course! as well as specialty bindings (broadway and hollywood) and other languages (French, German, etc.).
In your case, this may have switched to something other than the default/basic
To change the key bindings, follow Tools > Preferences... > Editing > Shortcuts and enter
cua, or Browse for another.