Abusive husband?
New partner who doesn't want kids?
Depression?
Feeling like this isn't the life she wants?
Feeling like she isn't what her kids need?
Who knows. But women are human, they have the same capacity for all the things men do, even if it's less frequent.
The difference I guess is if it had been a man, they'd have probably stopped looking long ago, assuming he'd chosen to leave.
My Mom left my Dad and we stayed with him, although she remained around and we saw her weekly at least. I don't think she's any better or worse than the many men who do it.
My heart breaks for these now adult kids because she's truly abandoned them, but that goes for all the people who's Dad's do the same.