When I was reading about pacemakers, it was implied that they could be accidently triggered by a number of things. The people that have these are warned by their doctors what the jolt they give is like so they will be somewhat prepared. Pacemakers can also be stunned into safe and default modes if their logic detects something is wrong. This may go unnoticed but they may not be able to operate as needed in such a fail safe state. There apparently are no official levels or standards of EM interference for these things.
Different brands appear to be affected more or less.
Obviously this situation could have been disastrous. Having a pacemaker zap someone would be sort of unpleasant.