Thank you so much @ManviSharma. I will look into page history and check if found exact cause.Can we write any service or listener to get exact cause?

We are not getting sorted list according to modified column on click of modified column in list view of sites.html due to below reasons,cq:lastModified property is not available/removed for few pages.cq:lastModified property miss formatted with date(2018-10-18) for few pages and for other pages date...