Continuity mistake: When Eric reaches forward to touch Top Dollar's face to show him Shelley's thirty hours of suffering, he reaches up with his right hand, but in the next shot, his left hand is on Top Dollar's face followed by his right hand. (01:29:45)
Deliberate mistake: When the crow drops Shelley's ring into Sarah's hand at the end of the film, the sound of metal upon metal is heard. (01:30:00)
Continuity mistake: In the scene at the bar with Gideon when the black guy pours Gideon a drink it is only about 1/3 full. When Gideon drinks it in the next shot it is almost full.
Continuity mistake: In the scene where Brandon walks in on Top Dollar's meeting and there's a big shoot out, the guy in the trousers with hearts on them gets shot and killed, but a few seconds later you see him again shooting at Eric.
Continuity mistake: In the scene where Eric kidnaps T-Bird and forces him to drive the hot-rod to the dock, Eric holds a large silver revolver up to T-Bird's head, then removes a black automatic from T-Bird's hand and drops it into the front floorboard. A few moments into the chase we see Eric holding the black automatic to the side of T-Bird's head, and the silver revolver is gone.
Continuity mistake: When Lee takes T-Bird in his car and the chase with the Police begins, the camera flashes to look at the chase through the eyes of the crow, the Police cars emergency lights are strobes, but when the camera reverts back to the chase scene they are the old style rotating flashers.
Continuity mistake: Watch T Bird's facial scars, they change from small to big throughout the police chase.
Plot hole: Eric and Shelly were killed because they were trying to fight tenant eviction. Yet, a year later the building is seemingly abandoned. Eric and Shelly's stuff is even still there. So what were they killed for if nothing was ever done with the building?
Continuity mistake: When Sarah is sitting at the Hot Dog stand near the beginning of the movie, her hood changes from being up and down without her touching it.
Audio problem: When Shelly is being wheeled away in the beginning, she makes a strained gasping sound right before they put the oxygen mask back on her. But her gasp doesn't match her mouth movements. In fact, it's impossible to make the sound she made with the mouth movements she made. Try it yourself. It almost looks like she's saying "F**k" or something similar, and they just dubbed it over with a gasp. Additionally, the gasp is far too loud and clear given how softly she's speaking. Bad looping.
Deliberate mistake: When Eric first enters his old apartment, watch very closely. When he first wanders in the room, his body has a slightly weird jitter and blur to it that doesn't quite match his movements. This is because the footage of Eric walking through the doorway is actually taken from the scene in the alley. They "cut" Brandon Lee out of that scene and inserted him into the apartment since they didn't have a shot of him. But unfortunately, it has some odd "glitches" due the footage not quite matching.
Answer: There is no explaination given in the films, they are just brought back when the crow feels it is the right time to avenge their murder.