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.
Visible crew/equipment: When Eric disappears for the first time the camera spins around on Albrecht. In this shot, the shadow of the camera can be seen at his feet. (00:31:50)
Other mistake: When the girl is kidnapped and Eric goes to save her, he enters a big door of that Gothic cathedral (as depicted on the movie cover). At that moment, the wind blows. But for the scene to make any sense, the wind should blow from the outside in, not from the inside of the church out, but the motion of Eric's hair shows the wind blowing the wrong way. (01:05:55)
Continuity mistake: The can Albrecht drops on the floor when Eric visits moves places without anyone touching it. When Eric says about the hat still being on it's on the wooden flooring. When he walks past to read the file a few minutes later the can is on the carpet. (00:43:25)
Continuity mistake: Top Dollar shows a snow globe to a female companion near the start. Her grip on the snow globe changes instantly. In some shots she has her fingers on the glass and not in others. (00:23:35)
Continuity mistake: When Sara is going to the abandoned building and takes off some wood pieces, the gap between the pieces increases between shots. (01:01:00)
Continuity mistake: When Eric puts Fun Boy in the tub and runs the water on him the shower head changes position between shots. (00:40:25)
Continuity mistake: When Sarah is skateboarding, and the guy in the car yells at her to get out of the road, she pushes with her right foot and has her left foot in front (a "Regular" stance), but in the next shot she is riding "goofy" stance (right foot front), with no time to change stances. (00:24:25)
Continuity mistake: Officer Albrecht gets out his car and takes aim at Eric from behind the car door. The camera changes and Albrecht is walking forward without closing the car door or going round it. (00:30:50)
Continuity mistake: Eric smashes the door to Gideon's Pawn Shop and stays outside. The shot shows Gideon on the floor then Eric about 6 foot inside the door. Not enough time has passed for him to get that far. (00:26:00)
Continuity mistake: When Gideon is in the bar, he takes the top off the bottle just as the other bloke grabs it. The second bloke then proceeds to take the top off the bottle again. (00:41:35)
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.