Hi K, sorry to read about your father, may he rest in peace.

Selective Neck Dissection and Hemi-glossectomy is fairly common procedure in treatment of Oral cancer, but unfortunately, your father seemed to have developed some complication. It is very difficult to say if what exactly would have happened as neck has highly complex anatomy.

You could look for some clues or answers in final discharge summary (collect from hospital if you have not done it yet) as it would describe the exact procedure performed, whether carotid artery or Internal Jugular Vein (IJV) were impacted by tumour etc. Nasogastric tube causing a rupture seems to be improbable. It could be also due to pre-existing cardio-vascular conditions, which probably went unnoticed.