Why would a widow run away after her husband, who was the previous mob boss, dies? And why would the new boss want her back for himself?
Is this a plot point in your own story? If it is, then why is it? It seems like you don't even know why this is happening. If you don't know why something in your book is happening, then it shouldn't be happening. Edit: The above comment was made when I didn't know that "discovery writing" was a thing, so just ignore it because it doesn't matter. If you want ideas, then, when taking over a position of power, it's pretty important that you kill all family members of the previous family in power. This ensures that the family doesn't come back to retake the "throne" in the name of their late family member. It will also quell any potential revolts by the little guys in the name of the still-alive family member.
Maybe she's done something or knows something which means she's a threat to the "new" guy in charge. If it's something she's done, then perhaps the only reason she stayed alive was because her recently deceased husband protected her. If it's something she knows, then perhaps she would use the information to extort money from the "new" boss. Because, lets face it, nothing will carry over to the widow, she will be left with nothing (except maybe personal money) but if she has no job or income, then she could potentially be of the mind that she needs to get cash from somewhere. Maybe she knows the new boss won't treat her right, maybe she doesn't like the idea of being passed down like a possession to the next boss, part of the office furniture?
Why does anyone want anyone? The heart (or wiener) wants what the heart (or wiener) wants. Because the guy that killed her husband wants her for himself.
That's not really quite fair. A lot of us are discovery writers, and questions like the OP brought up are simply a part of our process. We don't know why, until we've written it and then altered it to fit our character's motivations. It sounds like that's what the OP is doing here. ETA: to answer the OP's question: She runs because she knows too much, and the new guy wants her because of that and power. Winning her over would be the ultimate "Fuck you" to the dead boss.
I imagined it going something like this: the old mafia boss was talking to the cops to get a plea deal or WPP for both him and his wife. However, it doesn't work out. Boss is dead, wife on the run, and new boss is hunting. And what would be better than the new boss to knowing that he got everything in the end: power, money, and wife. The ultimate fuck you.
Huh, I just looked up "discovery writers" and I've never heard of them before. I dunno, guess I never thought that someone could write like that. But, yeah, my comment came from ignorance, so I'll retract it.