Because life is full of triggers from the past. They come riding in on the wind, or a particular angle of light on the trees, or certain sounds, words, textures, smells.
Our conscious mind is unaware that an older part of our brain recognizes a sensation from a past event, but suddenly we feel sad, or we get a sudden muscle cramp or stomach ache. Listen to a song and feel the feelings you felt the year it came out. Both positive and negative feelings can be provoked, but our brain is wired to look for threats to our survival.