heathrow approach stack question

[starliner]

Aircraft from the Ockham stack generally peel off and approach the field from the south. Occasionally though, esecially during the 6am rush, they peel off and fly north over the field and join the approach from the north, fitting in between LAM and BOV arrivals. Interested to hear if there is a "rule" as to when this is done, or is it based on the landing runway.
Tks