Flashbacks, which are involuntary, occur when we hear/see/feel/remember/etc. something we feel strongly about. The flashback can take the form of pictures/images, smell, sensation/feeling, sounds, numbness, etc. Sometimes, the Flashback can be associated with no specific or certain memory. You just feel it. Flashbacks occur in people who suffer from PTSD (Post Traumatic Stress Disorder). Fir example, a person who has went and been engaged in war often suffer from PTSD. Flashbacks often cause the sensation of panic, stress, fear, or hopelessness. Though they are just memories, they appear very real and to be happening in the moment; a very strong sensation. Flashbacks are necessarily always the result of trauma, though it is often the case. It can be caused by other feelings and sensations which remind of us certain memories from the past and could not be troubling at all.
Simply, Flashbacks can be a result of any STRONG emotion, be it happiness, sadness, fear, joy, etc. Whatever it is, the emotion is STRONG and very REAL, as if the person is re-living that moment.