Sample to reproduce:

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.RadioButton;
import javafx.scene.control.ToggleGroup;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Main extends Application {

    public static void main(String[] args) {
        Application.launch(args);
    }

    VBox root = new VBox();

    @Override
    public void start(final Stage stage) throws Exception {
        Scene scene = new Scene(root, 300, 300);
        final Label label = new Label("_Label");
        label.setMnemonicParsing(true);
        final Button button1 = new Button("Button1");
        final Button button2 = new Button("Button2");
        ToggleGroup group = new ToggleGroup();
        RadioButton rb1 = new RadioButton("setLabelFor(button1)");
        rb1.setToggleGroup(group);
        rb1.selectedProperty().addListener(new ChangeListener<Boolean>() {
            public void changed(ObservableValue<? extends Boolean> ov, Boolean t, Boolean t1) {
                if (t1) {
                    label.setLabelFor(button1);
                }
            }
        });
        RadioButton rb2 = new RadioButton("setLabelFor(button2)");
        rb2.setToggleGroup(group);
        rb2.selectedProperty().addListener(new ChangeListener<Boolean>() {
            public void changed(ObservableValue<? extends Boolean> ov, Boolean t, Boolean t1) {
                if (t1) {
                    label.setLabelFor(button2);
                }
            }
        });
        root.getChildren().addAll(label, button1, button2, rb1, rb2);
        stage.setScene(scene);
        stage.show();
    }
}