An Alley Oop can happen when a person goes to wipe their ass after taking a shit. As a the hand brings the toilet paper up to the anus, an extra little turd falls out of the ass, sticks to the toilet paper, and gets slammed back into the rim... just like in basketball.
Unfortunately, an Alley Oop usually means that the turd will be smeard around on your ass and probably require a shower.
I went to wipe my ass and gave myself an Alley Oop. Gross.