Since the APD is required to response to the infiltration of the Altis Penitentiary, rebels usually purposely start this event to fight the APD. Other times, a gang member gets sent to jail for over an hour, and his gang will set up for a jail break shortly after.
Once the alarm sounds, APD will respond to the event until the bomb is defused or repaired.
Starting The Jailbreak
You will need a blasting charge to start a jailbreak. You can buy bolt-cutters from a Rebel Outpost or Black Market. To plant to blasting charge, locate the large white dome with the vault inside (circled in the first picture below). Walk up to the the large door of the dome, open your inventory by pressing "y", click the blasting charge, and press "use". Unlike other events, there does not have to be APD officers online to place the blasting charge. For the blasting charge to be place, there must be two civilians nearby with a 5.56 caliber gun or above. The charge will count down for 15 minutes before it explodes.
If you wish to hack the anti-air system at the jail, you'll also need to buy a hacking terminal. When the anti-air is operating normally, civilians will receive an anti-air warning when the enter the big ring; then, the civilians will be shot out of the sky by a heat-seeking missile if they enter the small center circle. Normally, the APD can enter the anti-air without being shot. If the anti-air is hacked, these roles reverse.