In The Host by Stephenie Meyer, how did the male healer die?

In chapter 53, the humans take 2 healers unconscious back to the caves for Wanda to teach Doc how to remove the souls, one male and one female (Candy). Wanda guides Doc through the removal of the male healer's soul, but then the man dies. It says that he was on his knees on the cot, staring at them in shock and Jared had to hold him down. He stared at Wanda, said "Why?" and then his body fell limp onto the cot, with two trails of blood flowing from his nostrils. He obviously died, but how? What did Doc and Wanda do wrong during the removal? I really like this book but that part still confuses me...thanks :)