Let's rewind to last year, around the same time. Me and my horse were barrel racing, and he was brave. I could push him fast into the turns, with little resistance, and we'd do good. At one of the shows, a turn went terrible for whatever reason, and he slammed into a metal barrel. Now, where it used to be a little resistance, there is a lot of resistance going into turns. He used to slow down a tad going into the turns, but now, probably 5-10ft before the barrel he slows WAY down going into the turn. My best guess is that the metal barrel hurt him enough to scare him a bit from performing his best.
Horses usually knock barrels because
1) They are dropping their shoulder
2) Or the rider is turning them too soon.
Whenever a problem arises like this, it is best to go back to SLOW WORK and build the horse's confidence again.
A video would be very helpful to see if YOU are inadvertantly doing something wrong. Make sure you give him enough pocket in the turn, and make sure you keep his shoulder up, and make sure you do not ask him to turn until your knee is at or past the barrel.
It one thing to "sack out" your horse with a metal barrel from the ground, and it is completely another to have them tip over while you are riding him. I never think the second option is acceptable. I don't want my horses to know it is an option for that barrel to fall over (same with pole bending). From the ground you can bang on the metal barrel to make sound, tip it over toward your horse, roll it toward them, etc, to make sure they are fine with it from the ground, if that barrel becomes "scary" at any point.