I'm not sure it was voluntary. O'Brien's contract was up this year - actually in May of this year. Rather than lose O'Brien to ABC or FOX, NBC forced Leno out - remember, unlike Letterman, Leno didn't own the "Tonight Show", NBC did and still does. It would be interesting to know if NBC would do it over again considering O'Brien is getting beat rather soundly by Letterman.
Essentially, Leno didn't quit - he got fired.
However, nobody will have to hold tag days for either Lecherman or CBS......the entities involved no doubt have adequate insurance to cover all types of lawsuits including sexual harrassment and the creative spin-offs from that.
Leni