According to this article, Shanahan thought Marshall was a burden to the team, despite his election to the Pro Bowl.
Sources said Shanahan had grown weary with Marshall off the field and felt he was unreliable on the field
Shanahan, a source said, thought Marshall had cost the Broncos more victories with poor route running, dropped passes and tepid run-blocking efforts. The coach also anticipated some contract problems, a source said.
I never liked the idea of Marshall coming to to the Bears. I cited that his bad atitude would negatively impact the Bears' locker room. This news certainly confirms my fears. What's surprising is that Shanahan wanted to drop Marshall because of more than just bad attitude; it sounds like Marshall had bad receiver play.
Perhaps most telling of all,
Shanahan was not planning to shop Marshall on the trade market but rather just release him after the receiver played in the Pro Bowl.