Electrical – Electric Shock from concrete pad

MY band recently played a gig at a friend's farm. We set up on a concrete pad behind his barn. I received my first shock when I set my arm on the top of my guitar amplifier. It was a hot sunny day and my arm was a little sweaty. Shock was minor. I then touched one of my effects pedals with my hand to adjust settings while holding my guitar with my other hand and received a very severe shock that literally knocked me off my feet. Our bass player's amplifier (next to me) completely fried as did a replacement amp. Our other guitar player who set up on other side of stage had no issues even though he was using same cord/circuit. We changed power cords, used different circuits, etc. The only thing that finally solved the problem was by plugging our gear into the extension chords on the other side of the pad. In other words, the issue was somehow related to the place on the pad where we were setup. Have you ever heard of such a thing? In 30 years playing, none of us had ever come across anything like this. We are all pretty good at tracking down electrical issues. Thanks for your reply.