The reason they are on lockdown now is that some people were in the parking lot with a megaphone yelling "Austin wants Julia out" John, James and Meg were in the backyard when it was first yelled. Austin and Julia were with the other 3 when it was being yelled again.
They were being told over and over by BB it's a lockdown get in the house...I think Austin heard it, when they went in the house he sat by the counter just staring, in deep thought. This has been a good night for live feeds