The official results look like it was the breaststroke to butterfly exchange. The automated reaction time/relay exchange time shows a -0.04, indicating the butterfly swimmer had left the starting platform prior to the breaststroke swimmer activating the finish pad.
Swimmers are allowed to be in motion during relay exchanges, but the outgoing swimmer must maintain contact with the starting platform until the incoming swimmer has finished. The incoming swimmer's finish is registered by the touchpad on the finishing wall, as is the outgoing swimmer's contact with the starting platform. The two points in time are compared by the electronic timing system, and if the swimmer is off of the platform early a negative time is shown - and that means a disqualification because the starting swimmer left too early or the finishing swimmer mis-judged the touch and missed. There is a an allowable negative number, -.03, but the timing system shows a -.04 which is .01 over the allowed amount.
Great relay exchanges are part of fast swimming. They take fantastic timing, and there is very little margin for error. But being human, swimmers do make them from time to time, even at the elite level. Something to work on for next time.