Before you can determine why you need to determine whether this drop was gradual or sudden, a sudden drop could be due to your website having noindex tags accidentally added.
When sites are moved from a test environment to a live site, the developers sometimes forget to remove them. If they were left on the site, they would deindex your site. If your site uses WordPress, you can check for this under Settings, go to Reading, and search engine visibility. The box that says “Discourage search engines from indexing this site” should not be checked.
Malware, due to your site being hacked, is another issue that can cause your site to be removed from the SERPs. If Google determines that your site could be harmful to users, they will deindex it.
A gradual drop could be caused by a Google core algorithm update that has seen other websites gain traffic and rankings above your site. This change is done to help Google improve how their system assesses content overall which can lead to some page performing as well, while other that we under-performing may do better.
It is most likely that a gradual drop in organic traffic is caused by the fact that your competitors are using a more effective SEO strategy, taking over the organic visibility on Google, meaning they have earned some of the rankings that your site used to hold. It is best to perform a complete analysis of your competitor’s SEO strategies to find ways you can improve and increase your organic traffic.