Can range from anything like previous government employment ( police, army, internal intelligence, diplomatic posts etc even if your work there is just a civilian type), living or repetitive travel to unstable areas or war zones or countries that are known to harbor or be source of criminal activity, drug source countries, unexplained repetitive travel to a specific country even if a stable one ( where its obvious you have no ties there), being from certain nationality or religion, marriages from other religions or nationalities etc. Basically if the visa officer has the slightest doubt, he/she might request further security screening based on that red flag.