It could be besause u are not confident about the topic. learn ur stuffs well and when u go there think others are fools (thats doesnt mean u can speak what ever )and just talk about all that u know u will be good. and remember even if u make mistake ppl may laugh at u once but no one is going to remember ur mistake the next hour so dont worry.

Thats one fear everyone has at some point or the other but as u give more and more presentations u will get more confidence.

Good luck.