QSO's using FT8 are often fast and furious. When conditions are favorable it is easy to end up in a pileup situation with many stations calling you at once. WSJT-X includes a helpful feature that enables the software to automatically respond to the first decoded responder to your CQ. Just check the box labelled, "Call 1st".
This is very helpful to keep up with the pace of QSO's. It is not intended to be a substitute for an operator actively controlling their station.