Some blokes are modest and will cover their cock with a towel on the way to the shower but leave their beautiful ass exposed for admiration and contemplation. On their way back to the lockers, they'll wrap themselves in the towel. Once they start to change, they'll turn toward the locker to hide their cock but leave their naked ass exposed. When they bend to pull on their underwear (or pants if they're flying commando), you'll get a quick glimpse of their vulnerable hole.
Some lads don't care if everyone sees them naked. Chances are they're hung with a bigger than average cock, too. They want everyone to see their weaponry, even if they are straight. They'll wander all over the locker room, bend down in front of other blokes to show off their joy chute, and take their dear sweet time getting dressed. It's the safe place to be an exhibitionist.