The problem is with the "each" loop. Removing items in the loop is changing store size and hence only some of the items are getting deleted.
To get around it, you can loop through the store in reverse order and then delete as below:
var grid = new Ext.grid.GridPanel({
width: 400,
height: 600,
store: store,
loadMask: true,
renderTo: Ext.getBody(),
tbar:[
{
text:'Delete all type = delete',
handler:function(){
var i = store.getCount()-1;
for (i; i>=0; i--) {
if (store.getAt(i).get('type')=='delete') {
store.removeAt(i);
}
}
}
}
],