Revealing mistake: When Erin gets into the truck, in the next shot the camera backs up a little and you can see that it's only raining on the truck.
Revealing mistake: When the kids are in the van and Leatherface is on top with his chainsaw, you can see in one cut that the mad magazine poster/cloth on the roof is already cut, just before Leatherface actually starts sawing into it.
Revealing mistake: When Erin runs over the sheriff, she is shown bouncing around, holding onto the wheel with both hands. As she's driving away, the camera pans to the baby, who's sitting straight up on a blanket (?) with no seat belt on. Highly unlikely position after all that erratic driving.
Revealing mistake: After the girl has shot herself in the teen's van and you see through the hole in her head, you can see it is a dummy when it falls back. (00:10:30)
Revealing mistake: On the three drivers licenses, the date of birth and address is the same on all of them. Also, their birthdays read 3-11-92, but the movie is set in '73.
Answer: Erin must have led the police back to the house and then the Hewitt's were taken away so that way evidence could be collected. They must have thought that Leatherface had escaped and was no longer in the house.