I don't think we know Sarah's real reasons for stepping aside. There's no doubt in my mind that she was running an undeclared presidential campaign, right up until about two weeks before her announcement.
She was on the path to the White House, but something caused her to stop. I know what she said, but I don't buy it. If her family were opposed to her running, she would have let the country know much, much earlier that she wouldn't run.
Sarah has served in one public office or another for about twenty years. She's been a wife and mother since day one of her political career. That never stopped her before, and I highly doubt that it stopped her this time.
I though it was odd, too - but I doubt it was any type of threat, which would have just brought out the Mama Grizzly in her big time.
Maybe when she asked God to "search her heart" He revealed to her His will for her, and it wasn't to be POTUS (not right now, anyway).
Do not doubt for a minute that God is in control - He is still Sovereign over all - and His plan, whatever it is, is perfect.