Some call it an attitude problem others call it a hunger for playing time and wanting to win. Personally, as he's a 2 time All-star and an 1100 game veteran I tend to believe it's the later. Also, take what you hear from the Cubs side with a grain of salt as currently, they are a mess. BTW, Montero owned up to his comments and personally apologized to Arrieta.
"Leaders who don't listen will eventually be surrounded by people who have nothing to say"