well it jus happens like that. i have been out today i have been wareing my jeans that hang low well jus sit on my hips or what ever they seem to be held up by nothing. and my boxers we in the perfect position this morning. but as the day went on they move upwards and things get tangled and you dont get time to re arange. but my jeans arent aslow as they show full on arse like you describe its more of boxers ride sightly to high. as for my work trousers well as the night goes on they end up under my arse flashing lots of boxer short.