Near the start of Neverwhere Croup and Vandemar have cornered Door. She uses her powers to open a door in a wall using the last of her energy. The relevant quote is
'If it's the last door I open' she prayed, silently to the Temple, to the Arch. 'Somewhere... anywhere...safe...' and then she thought wildly 'somebody'.
I understand why the door would take her to London Above as it is further from Croup and Vandemar although by that logic it should take her to China or something like that. The fact that she appeared next to Richard suggests the door was aiming for him. If this is the case why did the door take her to Richard and not someone who could help her like Marquis de Carabas or Old Bailey?