Here's why:
* Adjective: It describes a noun, meaning "involving or characterized by questions."
* Example: "She had a questioning look on her face."
* Verb: It means "to ask questions" or "to doubt."
* Example: "The police were questioning the suspect."
While "questioning" can sometimes act as a noun, it's not its primary function. In those cases, it typically refers to the act of questioning itself.
* Example: "The questioning of the witnesses took several hours."
However, even in this example, it's more accurate to say it's a gerund, a verb form acting as a noun.