Someone who keeps to themselves but listen to everyone else's problems. On the outside, this person is quiet but cautious. On the inside, this person is dealing with conflicts (feeling lonely, unappreciated, worthless, etc). This person never let's people in, so expressing any feelings is uncomfortable. Once in a while, this "time bomb" has no more room for emotions, exploding into anger and tears from frustration.
"Susan won't let anyone in. She's bound to explode like an emotional time bomb if she keeps everything inside like this."