I’ve been wondering this myself for awhile as well, because I can’t even pinpoint a direct cause for my emetophobia, I was age 3 when it really started for me as well at a early age. I used to be sick a lot as a kid too, especially with stomach issues and viruses. I’d have anxiety so bad I’d get sick in elementary school constantly and would have to go home as soon as 10 in the morning. So I do think it has something to do with anxiety disorders, mine is very much related to Ocd and the need to feel in control of myself. Having consistent horrible experiences with being sick also contributes to that feeling of terror similar to a response someone would give to trauma even, there’s so much you could go into. I truly wish emetophobia would be taken more seriously and given the research it deserves because it’s such a chronic and life-altering disorder(I believe it should be considered one) that’s very misunderstood or belittled