The System Purge, also called just the Purge or the Carnage, was when Marty Mikalski and Dana Polk unleashed all the monsters in the glass rooms into The Facility. This was accomplished with a button in the control panel marked "System Purge."
During the Purge, each and every one of The Facility's monsters are let out in waves, eight at a time. The first wave of monsters included: the dragonbat, the Angry Molesting Tree, the zombies, the Dismemberment Goblins, the Giant Snake, the killer robot, a witch and the werewolf.
Why the facility had the need for a System Purge is unknown, but it is likely that this is a reference to the eleventh-hour deus ex machina that is a common trope in the horror genre.
Ironically, being such an advanced corporation, the Facility was very ill-prepared for the purge. The only reinsurance of safety was on security, but, even they weren't able to fend off the attacks.
It can be safely assumed that the Facility had never faced a problem like this in the past, or a mere possibility stands that they hadn't faced such a problem like so in a while, but even if this stood true, there at least would have been stronger security. But, during the event, many attempts to futile the outbreak were attempted in Command. But, it can be assumed when Marty and Dana were wiring access to the elevator, that the wires they cut were important to the system's security access and procedures.