Why does the reset button on a wall outlet pop out when something is plugged into it?

Assume this is a ground fault circuit interrupter (GFCI) outlet. The GFCI looks for current that is flowing to ground, in a manner that could result in electrical shock to a user. The appliance that is being plugged in could be defective, the wiring connected to the load side of the GFCI may be incorrect, or the GFCI outlet could be defective. Try using another appliance. If the GFCI holds, the first appliance is most likely the cause.