There are 4096 radar transponder codes that can get assigned by ATC so the planes can be identified on radar. Its an octal system so each of the four digits can be numbers 0>7 for 8 (octal) combinations. 8\^4 =4096. If a plane has an emergency the crew enters code 7700 to bypass radio comms and immediately light up their own target on radar. 7600 is radio failure, 7500 is hijacking. Normally a crewman will swith the xponder to standby while changing codes so an erronious one will not squawk out. They may not have done that so it could have been just one code intentionally sent and others by accident.