A method is getting called unnecessarily

For some reason, getmycust() method is getting called as soon as the page loads. Can somebody please tell me where am I going wrong.

<apex:page standardController="Reservation__c" extensions="CarSharingClass">
<apex:commandButton value="Show Fahrzeuge"  action="{!showPanel}" / >
// I want to show the table only when the button is clicked.
<apex:pageBlockTable value="{!myCust}" var="showFahr" rendered="{!If(myCust != null && myCust.size>0 && isRendered==true ,true,false)}"  >
</apex:pageBlockTable>
</apex:page>

Class

public class CarSharingClass {
 public PageReference showPanel(){
        isRendered = true;
        return null;
    }
    public List<Fahrzeuge__c> getMyCust() { 
}
}