I think it's the implicit pack mentality amongst men contemplating sexual violence against a woman. Those guys invited some other men to join them in a rape, knowing nothing about them other than their sex & approximate age.
They weren't afraid the strangers would call the police, they assumed willing complicity - and they were right. It's similar in quality to those events where a man, observing a rape in progress, doesn't try to stop it or call for help but says "Can I have a go?" The unspoken bond between the men as rapists holds good, although they've never met before.
NAMALT, obviously and thank god.
In combat situations, we know that a group of rapists will goad & threaten any man holding back or showing disapproval. Enforcement of the male bond through sexual violence takes priority over all other concerns.
I can think of some other kinds of violence where a similar mechanism operates: it's happened in football violence, racist, homophobic and ageist beatings. It's quite possible that an anti-immigration male in a supportive environment could successfully invite strangers into a room to join him in beating up an immigrant. A soldier with a captive enemy certainly could.
Those are both political motivations, with the aggressors assuming a bond through being on the same 'side'. But random blokes on holiday? The only thing linking them is their sex, and the sex of the victim. It speaks to a near-universal and universally recognised male bond through hatred of women.
I can't think of any equivalent amongst women - do correct me if I'm wrong!
Women might bond in violence against a certain person deemed awful. But not because of a global characteristic like sex, or even race. It might be because they think he's a child molester; not merely because he's a man.
I can imagine women successfully enjoining female passers-by to take part in a robbery, say in a similar hotel where they'd found a room full of Chanel goodies or iPhones. Then, too, men could do the same. That's a bond of temporary greed, if you like, not a general bond of half the human race against the other half.