How do I change the default value of the “position” category attribute? in magento

You can change directly in this method Mage_Catalog_Model_Resource_Product::_saveCategories
Somewhere in there this is this:


$data[] = array(
    'category_id' => (int)$categoryId,
    'product_id'  => (int)$object->getId(),
    'position'    => 1
);

Replace 1 with 100.