Really a lot does but here are the main things:

Crowds--especially in smaller areas, I can handle them in wide open places but when I get confined, I start to sweat and feel trapped.
Embarrassment--anything big or small and especially in front of a lot of people.
New social situations--this has been the one thing that has hindered me since I am older, about to graduate college. I really have intense anxiety about doing things without people I know. I do not like going to events where I do not know anyone. So it has kept me out of a lot of social events, clubs and etc. I have a job but I work with someone I knew before the job. I really fear that this will affect me after I graduate. I hope I can overcome it.
"Risky" nights out-- my best friend is very much into nightlife. I enjoy having fun with friends but I find most situations she has found herself in to be horrible to me. I would never want to go out again if I experienced what she had. I fear everything that could go wrong. I like low key nights with friends and my boyfriend, I have been out before and do not see how it is so amazing, it causes me more anxiety than anything.
Future--because the future is unknown, I really get anxious when I think about it. Again I fear my anxiety will cripple me in the real job world. I am trying my best to overcome it now. I hope to find an internship before I graduate. I worry about all of it. That I will not get into graduate school. It all went by so fast. I worry I am not prepared.
Sickness, Death, Disease, weird feelings-- I really worry when I am sick, or have "weird" feelings, I get worried when loved ones are sick. I always fear the worse. If I have a weird feeling I think that something is really wrong with me. Family members always joked that I was a hypochondriac, but I think it is true. I worry about every twitch and tightness or anything that I feel going on. I worry about medications and germs. I also have really severe allergies and I do not know if that causes this fear of "environment".

I have a lot of triggers especially in social situations and anything unknown. I have been improving after working with a counselor, but I have new triggers. It is a process, a journey to relief I hope. And I hope you find that relief as well.