My guess. Pure guess. Uneducated guess.
Conventional wisdom is to start a scene late and quit early. Say for example the scene is a party. The MC can be there ASAP, witnesses all irrelevant happenings, numerous bit-role characters come and go, before the murderer shows up, kills MC's wife, and leaves. Alternatively (starts late), the MC can show up right before the murderer does, just enough time to introduce his wife to the host.
After the murderer has gone, the MC can stay, mourns, goes to bathroom, calls his family, gives statement to cops, informs friends, have midnight snacks, etc, before calling it a night. Alternatively (leaves early), he can jump onto a car and chases the murderer (another scene), or he passes out when wife dies.