This summer, Ken (along with CNU and Eunkwang) performed in the musical stage production of Hamlet.
Only, Ken didn't get to perform that much because all the actors would go on strike the day the show was about to go on. The audience would be told to leave without much explanation and the show was cancelled that day.
This only happened for Ken's dates.
Understandably, fans were upset, but I can also see the actors' point of view. They were on strike because they were protesting the constant use of idols in musicals.
Because when an idol is cast in a musical, the theatre sells more tickets, it is now rare to see a musical without at least one idol in a leading role.
That means musical actors who did not go the idol route (perhaps they worked their way up through small theatres, went to theatre school, etc) are missing out on big roles. Even if another actor is immensely talented, an idol will be given the role instead because they will sell more tickets.
It was nothing personal against Ken (the actors made that clear) but they needed to make a stand somehow, and Ken is a big enough name that it would get people's attention.