A patient with a large mediastinal tumor presents for excision of the mass. CT scan shows a mass of 14 x 10 cm causing about 40% tracheal compression and moderate compression of the superior vena cava (SVC) and other mediastinal structures. The heart is displaced laterally and an apical impulse is felt just medial to the anterior axillary line. The patient is asymptomatic with no signs of SVC syndrome. Which of the following is the MOST common mechanism of hemodynamic collapse under general anesthesia in this patient?

A) Increased afterload
B) Decreased preload
C) Ventilation-perfusion mismatch
D) Rhythm abnormalities