What is the order of preprocess functions?
The order in which the preprocess functions are called in Drupal 7 is 


template_preprocess
template_preprocess_HOOK
MODULE_preprocess
MODULE_preprocess_HOOK
ENGINE_engine_preprocess
ENGINE_engine_preprocess_HOOK
THEME_preprocess
THEME_preprocess_HOOK
template_process
template_process_HOOK
MODULE_process
MODULE_process_HOOK
ENGINE_engine_process
ENGINE_engine_process_HOOK
THEME_process
THEME_process_HOOK


Source : https://api.drupal.org/api/drupal/includes!theme.inc/function/theme/7