Google might have flagged your site for spam because of automated checks, reports from competitors, or user complaints, highlighting the importance of sticking to Google's rules and fixing any issues they point out
In my day job, I head an operations team so I am big on checklists and SOPs. I have checklists to ensure that I am on the right side of Google. So my guess is either I was reported or the automated checks got it wrong. I looked at the reasons for the manual action and my site does not violate any single item.
Even though you follow all the rules and have checklists and Google gave your site a penalty. It might be due to mistakes in their checks or someone reporting. If your site is legit, try appealing.