Granted, this is just a theory, but it's plausible. When Zazu said that Nala was betrothed, he meant when she came of age. Likely Mufasa may have died before that happened or Simba would be of age too and so she'd marry him. However, if Simba was dead, maybe she was scheduled, via the betrothal law, to be made Scar's mate by the Pridelands law on what would be her 18th birthday, so she left right before that point.
It would also explain why she left alone, as she was technically planning to break the betrothal law. Otherwise, it would seem odd that she just went to get help by herself.
Perhaps Scar was planning to make Nala be his main bride, but when she went AWOL, he took Zira instead.
If this theory overall is true, then Simba and Nala might have just come of age around the time that they returned to the Pridelands.