There are two types of Google penalties, automatic and manual action. If the webmaster is an amateur in SEO in the majority of the cases, he will never have a clue and may never recover from an automatic penalty. That said, the hit came because the webmaster did not comply with the Google guidelines. In the case of the manual action, the webmaster gets a message in Google Search Console that he has violated Google’s Terms of Service. This time the webmaster has done something intentionally, and has a hint of what to fix as the message doesn't give any instructions.
The cause of the manual action could be a redirect that went wrong, schema portraying false data about the page, i.e., showing fake five stars reviews, buying/selling/exchanging links (link schemes), doorway pages, malicious behavior (active or passive, i.e., hacked site), thin content, keyword stuffing, hiding text, etc.