We have experienced a shift in SERP from internal pages ranking over website homepage for more than a year. Previously website homepages used to rank for the primary keywords. Now we can see that internal pages like website.com/what-is-primary-keyword been ranking for the primary keyword . Google is picking up these "what is primary keyword" pages than the homepage. All our competitor sites are ranking with these internal pages which are about "what is (primary keyword)". We do have the same internal pages "what is....", but this pages is not ranking; only our homepage is ranking. Moreover we dropped more than 15 positions after this shift in SERP. How to diagnose this?