Selected answers from the Dumb SEO Questions Facebook & G+ community.
Thai Bui: What techniques do you use to figure out a significant decrease of organic traffic on a landing page?
SEMXE: See, if you have any recent changes. Or is the content is updated with current requirements or not? I mean see if the content is not outdated. If above two reasons are not valid, then try to run a survey on websites and ask your visitors of they found what they were looking for when they exit.
Thai Bui: Thank you for the advice. Recent changes is a possible culprit. Also, checking Google's algorithm history change helped.