The following is an excerpt from page 16 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding Combat Resolution, specifically the section titled, "OVERRUN".
There seems to be a bit of confusion regarding when a unit which is making an overrun move can wheel to charge an enemy. Put simply, when a unit overruns it will move directly forward. If this forward movement will take it into the enemy then, and only then, is the unit allowed to wheel to try and bring as many models as possible into contact. See the diagrams below for an example of this.