The quality of your content will always trump SEO optimization in the long run. Remember, SEO is what brings people over to your site, while the content is what makes them come back for more.
However, the spike you’re showing here is so abrupt that it may just be the result of a fad or a fluke (such as someone with a high profile talking about or sharing your site). You should monitor the graph over the following days, and see if the trend persists. If it does, then you can safely assume that the content is causing the spike.