When vampires go to war, whether between clans and sects or against werecreatures and mortals, like any other army they need foot soldiers. Lots of them. And fast. So, like any other army, they go recruiting. The result is a Mass Embrace, when vampires begin more or less grabbing anyone and everyone they can, right off the street or whatever, and turning them into more vampires.
Normally, most clans are very choosy as to whom they embrace as new members, looking for mortals who either already fit the clan archetype, or who have something that clan needs, whether it be skills or resources or influence. Most clans also tend to restrict their recruiting so as to not draw attention to themselves and attract raids from the Crusader Teams or other enemies. But in a Mass Embrace situation, these restrictions and requirements are ignored. Ugly and unstylish mortals may become Toreadors. The handsome and beautiful may become Nosferatu. The sane may be embraced by a Malkavian.
Generally, the clans of the Camarilla only rarely resort to a Mass Embrace, whereas those of the Sabbat consider it a regular tactic and will resort to such means almost at the drop of a hat. Within the Camarilla, the Brujah and Gangrel are most willing to use Mass Embrace tactics, in fitting with their role as warriors and fighters. Either way, those taken in a Mass Embrace rarely last long as Vampires, and are fairly universally regarded as disposable and expendable assets. Those few who do survive to become long-term members of a clan are often among the hardiest, strongest, and most ruthless vampires out there.