Book your flight in incognito mode
When you are checking for airfares, make sure that your browser is in Incognito mode. Airline websites store your cookies and this actually gives them information on your travel plans.
Source: BestLife
Once they find out when you want to fly based on the dates you search, they will raise the rates on those dates automatically. If you search for flights in incognito mode, it prevents them from using this tactic.