Easter shopping and Potty Training

A few weeks before Easter I took the boys to the mall to get there Easter outfits. I had it all planned out in my head. I had 3 stores to go to. Things went really well in the first 2 stores then we hit store #3 The Childrens Place. Evan had reached his limit. I was looking at ties that were near the front of the store with Evan right next to me and Cole in his stroller. I put a tie back, look down and Evan is gone. Then I notice that the big display "SALE" sign in the front window display was swaying back and forth. It was Evan running under the sign like it was a big sheet hanging just for him. I gave him a warning but it was just too much for him. Minutes later he was back under the sign. At that point a sales lady came up to me to let me know that he shouldn't be doing that because the sign could fall. (Apparently it had a few times already that day). I decided to take a break from the store and come back later. Evan had to hit the sign one more time and of course it fell. The sales lady was not happy. We left the store for about 15 min. Evan calmed down and i felt it was safe to go back. I was wrong. Evan was horrible. They have these big wagons in the front window displays and Evan was determined to ride on them. I pulled him off a couple of times but then he started taunting me and the next thing I know I am chasing him around the wagon in the window display. Litterally running after him. At one point I stopped and realized that I was "THAT" mom. My 2 year old was out of control. I eventually nabbed him but it took awhile. I quickly tried to grab what I needed so we could leave but I wasn't fast enough. Evan darted under one of the display tables and I could not reach him. He wouldn't come out. It was terribly embarrassing. He eventually got close enough to the edge so I could drag him out but it was the worst shopping day I have had with Evan.
Now on to potty training. How does one not know when they have to poop and pee? I don't understand. You know where you are suppose to go so why can't you get there in time. I don't get it. Evan is just not getting it and it is frustrating because I don't know what else to do or how to explain it to him in different terms. All I know is I am done changing his diapers so I will persevere until something clicks and he starts to go on his own.