wpf datagrid cell getting focused
===

I have a WPF tab control, and in each tab I have a datagrid.  
The problem is- if I select one datagrid item in one tab, after I switch a tab, the first cell in that tab (the second one) gets focused (-it has a black border around it).
![Focused cell][1]


  [1]: http://i.stack.imgur.com/7uyjG.png

Any idea why this happens?  
Thanks