What caused this?

that carb size is fine. it needs more fuel volume. It will run fine doing blasts on the street because it can use up the fuel in the bowls and play catch up. then a long full throttle blast is too much for it. Everyone always thinks they need a giant carb. The CI/rpm charts for carb size work well. I would get rid of the 5/16 fuel line and get a high volume mech pump and you wont have to worry about it again even with a few more upgrades.