Engagement spikes is something that is unavoidable on Instagram these days. Instagram is very saturated and every time they update their algorithm, your engagement will suffer (less people see your posts). Not to mention, Instagram Stories have taken away from post engagement.
Also keep in mind, we cannot force real people to like your posts so you have to make sure to keep testing your content. As a matter of fact a lot of influencers are showing 50% drops in engagement. Here is personal message about engagement: