In a drawn combat a Dreadnought can move up to 6" to attack unengaged models or turn to face its attackers if being assaulted from the side or rear. Any unengaged attackers can move in to surround a Dreadnought in order to attack its sides and rear (if there's room).
Drawn Combats
Source: Core Rulebook 3E, p. 91
In a drawn combat a Dreadnought can move up to 6" to attack unengaged models or turn to face attackers who came from its side or rear.