My guess is that she has read the tea leaves from the early voting and realizes that Trump is going to be re-elected which makes her former position untenable...:-)
“Trump just told the Press Pool that “The early vote isn’t what the democrats expected it to be. They aren’t turning out. Numbers aren’t matching up.”
Here on the Senate floor in her own words...:-)
https://videos.files.wordpress.com/AVVqER7n/lisamurkowskiyes_hd.mp4