Pre approved for mortgage loan, then absurdly denied...

My wife and I were pre approved for a mortgage loan with USAA. All our financial information was clearly explained in black and white, no gray areas. With that said, we made an offer on a home we were interested to buy since we were assured that we would be backed up by USAA financially, after all we had already been pre approved. Everything was going smooth and fine, seller accepted our offer, the home was well below the amount USAA had pre approved when suddenly to our surprise and after a home inspection had been done and a earnest deposit made, USAA decided to pull back. The reason for denial was based on some of the information that had already been disclosed during the pre approval process. My question and report of a problem is why would a respectful company such as USAA would mislead people in such way... What's the point of the pre approval process??? make customers think that they have been approved and then cutting them off... Even though I really like USAA I'm considering to move to another banking institution... this was a complete waste of my time and personally I felt humiliated....