What prevents reRendering of the whole Visualforce Page when changing between ?

in my Visualforce page I have created an apex:pageblock that consists of an apex:selectList for sorting and an apex:tabPanel (Leads & Accounts)

Now when I switch between these tabs the entire visualforce page reloads which includes a google map. This not what I want and at the same time can't figure out why this is happening. What 'action' tells the page to reRender when switching tabs?

I only want the pageblock to reRender. This behaviour is fine when I change the value of the 'selectList' element in order to sort the records inside the tabs.

Here is the code that handles the pageblock:

  sortieren nach:        <!-- TAB LEADS -->    
  <style type="text/css"> .col1 {width:5px; text-align:left;} .col2 {width:400px; text-align:left;} </style>    
 <!-- Jump to location and highlight -->         <!-- Distanz -->  {!distances[war.id]} <!-- Anzahl Anzeigen --> {!war.AD_MS_Rel_Anzahl_bez__c} Anzeigen <!-- Branche --> 
     
 <!-- Jump to location and highlight -->         <!-- Distanz -->  {!distances[pote.id]} <!-- Anzahl Anzeigen --> {!pote.AD_MS_Rel_Anzahl_bez__c} Anzeigen <!-- Branche -->