In Harry Potter and the Half Blood Prince, when Bellatrix encircled the Weasleys' house with a ring of fire, that's how the events took place
The Burrow gets encircled with the ring of fire (with a small opening in it).
Bellatrix Apparates in front of the door and ran out of the ring from that opening (mentioned above).
Harry comes down and then out of the house and follows Bellatrix.
Lupin and Tonks run after Harry to stop him but fail to do so when the ring (of fire) closes behind Harry.
Lupin and Tonks try to fight (or control) the ring when Ginny comes running out of the house.
The ring presents an opening again and lets Ginny cross it (presumably leaving behind the others i.e. Lupin, Tonks and Weasley parents to control the fire).
Why did the ring of fire open itself for Ginny, when the main purpose of the attack seems only to get Harry out of the safety zone?