Carson apparently had a number of malpractice suits and settlements—though about average for the number of surgeries he performed.
http://www.theguardian.com/us-news/2015/may/04/ben-carson-malpractice-claims-doctor-for-president
Carson was a premier expert (if not the premier expert) in pediatric neurosurgery. Which means he was called in to handle (and consult on) the toughest of tough cases. Very high risk surgery means things can go wrong, and when things go wrong, people sue (regardless of whether the surgeon was actually at fault).