Johnny has prepared and practiced an excellent presentation, but he's so convinced that he will get nervous that he actually gets himself nervous before and during his presentation. this exemplifies what common cause of communication anxiety?

I believe the answer is negative thinking
negative thinking refers to the type of communication anxiety caused by constant anticipation of the worst-case-scenario to happen in every situation.
People who displayed negative thinking are commonly also had lower self-esteem and make the negative anticipation become self-fulfilling prophecy for them.

 A population graph with J-shape growth curve represents exponential growth (growth without limits), in which growth rate becomes faster and faster as the population grows in number or size. In exponential growth, growth rate is directly proportional to the population size or number