All three of the actors you mention will have been with their shows at least a year once rehearsals and in the cases of Wicked and Little Shop, out of town engagements are factored in.
A year in a show is a healthy amount of time and it is better to leave while you're looking good rather than pulling out a performance like a wisdom tooth.
With all respect, Hunter and Kristen are not big stars, but Hugh Jackman, post X Men, certainly is and for him to give a show a year is a big deal. I hope he plays LA with it so "they" can see what he can do.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher