Jim Cramer's advice to trim positions in Disney stock is based on the stock's recent performance and the ongoing proxy battle with Nelson Peltz. The stock has surged 25% this year, but Cramer questions the sustainability of this growth. The proxy battle and management's response to a box office slump, including project cancellations, add to the uncertainty around Disney's future performance. Cramer's influence and the public nature of his caution could lead to short-term negative sentiment among investors.