It's apotropaic.
The cans ( and the confetti) represent the weapons thrown by the woman's relatives at the man who has abducted the woman from her tribe.
Sort of leftover from the Stone Age
Another weird old custom at weddings - as the newly weds were driven off in a car, the groom would throw a handful or two of pennies (old money!) out of the window. The local kids were expecting this and scrambled to pick up the loot.
Was this just a Geordie thing? We called it the 'hoy oot' (throw out)
Won't happen now - elf an' safety.
jacklee