Why are stores closing so much earlier than designated store hours?

I went to the AT&T stand alone store in Pasadena, MD 21122. They were closed before closing time. We left and went to the AT&T store in the Marley Station Mall, Glen Burnie, MD next. The mall store shows open until the mall closes. We were at the mall at 7pm and the store was already closed. The lights were on, but the gates were closed but not locked. Open about 3-4 inches. If I wanted to, I could have gone in. I stood outside for a bit and called the store. I heard my call ringing in the store, but no one came from the back or otherwise. Why are stores closing before the advertised hours for closing? All the other cellular stores were open and well staffed. ATT lost a lot of business that night I'm confident of it. Too easy to go to another open store nearby. I thought you would like to know this. That's a pretty significant infraction I think when you trust your employees to man your stores. If there was an emergency of some kind, there could perhaps be a quick handwritten sign? But then again, don't want to leave the gates of the store in a mall unlocked and roll out....