I assume that she's seen how Trump supporters treat Republicans who dare to contradict their Dear Leader, and she simply decided that she'd rather step away from Congress and potentially receive fewer death threats. Nobody knows the lengths a cult will go to better than someone who has just escaped.

If I'm being extra generous, I could say maybe she knows what happens to representatives who don't toe the party line, so she stepped down to avoid cuts to her constituents' services. But that might be giving her too much credit.