How does a sausage party happen in real life?
People are invited, but some don't show and others crash, skewing the ratio.
The girls that actually show are swarmed by 3, 4, or more to one, giving them a certain power over the mass of guys.
Some of them get overwhelmed and leave, others make their choice among the plethora of options and leave with that partner.
At some point, all the girls are either gone or 'taken' and you're left with a sausage party.