Theft
If a pilot picked up something from your jettisoned container, or a loot container dropped by an npc you killed, this also starts the 15 minute aggression timer. The container will have appeared yellow to the pilot at the time. If you are in a player corporation1, this also applies when the pilot takes something from containers of your corporation members. The pilot will have a special marking (blinking red background color by default, same as -5 security status outlaws). The pilot is flagged to you and all your player corporation members.
i.e. if your can is tipped, tipper is flagged to the whole corp and becomes a legal target for 15 mins. Allowing them to be attacked with impunity without you being criminally flagged.
So
NO ONE except the tipper can reply to your defensive actions as they and they alone become a legal target, even if they are in a gang the other gang members
CANNOT reply.
This does not apply to gang members.
i.e. Member A is in corp Y, Member B is in corp Z, Member B drops a can which is then tipped. Member B and all Members of corp Z may attack tipper but Member A and corp Y may not, as the theft was from a can owned by a member of corp Z.
Aiding a flagged pilot
If you provide any type of aid to a flagged pilot, you will receive the same flags for 15 minutes since the last act of aid. For example, if you aid a pilot at war, their enemies will be allowed to attack you. If you aid a criminal (illegal aggression or -5.0 security status), you are considered a criminal and their victims are allowed to fire at you as will CONCORD and sentry guns in the proper systems.
Shield transferring, remote repairing, remote sensor boosting, drone repairing, etc. are all forms of aiding. Basically any activity that 'heals' or 'improves' your target's ship are a form of aiding.
If the tipper does have help i.e. remote reppers they will become flagged also but they
CANNOT open fire until a defensive aggression act has been taken against them. i.e. one of us fires at them, but once again only they can reply to this as they are the law breakers and we have a roght to reply to any act of aggression legally.
Hope this clears things up