1) The national media narrative on Warren became more critical once she became a perceived frontrunner;
(cont)
3) Bernie went back on the trail and got some high-profile endorsements & has been sharp;
4a) Buttigieg went after Warren from the right;
4b) Buttigieg began to get more press coverage citing rising polls;
6) Last, and certainly not least, the track record of what happens to women in Democratic primaries once they're perceived as frontrunners also needs to be considered.