The big X from the machine just means the dude at the desk has to sit up and pay attention, because something tripped the automatic gubbins.
2 of our lot got X's on our last ski trip to the US, and I asked what it means.
He said it was just a flag that something needed checking, but it was not always clear what the something was.
Typically, it might be that the person had not previously entered the US under the same passport and ESTA.
So if you have got a new pp or new ESTA since your last entry, you will likely get flagged.
Or it could be something worse, involving bridge trolls, of course.