Tpye keyword…………. Keywords require how much domain authority and backlinks. If someone have all these things greater thn you, he can easily rank. Also one more thing I want to add. What is bounce rate for this page, if it’s high Google may think low and lower it’s position. Hope this may clear your answer. Or maybe some other reasons are possible too
Stop chasing rankings for individual keywords. There’s always flux and either way it’s pointless because you can’t control the SERPs for individual keyword rankings.
Instead, focus on overall, organic traffic growth to a page. Because if traffic is rising for a particular page, it means that the corresponding keywords that it’s ranking for are also rising higher/gaining more visibility, etc.