When a guy/girl un hooks a bra that contains very massive boobs. When the bra is unhooked the boobs come flopping down leaving u stare in amazment just like an avalanche....except with boobs.
Mike: DUUUUDE!! Last night me and my bitch were goin at it and u no how she has huggeee tits? Well when i unhooked her bra everything came tumblin down!
John: Mike my friend, thats what I call a boobalanche!