"Vorkommen" would be a synonym for "passIEren." The word "passen" means "to fit" or "to be acceptable," while "passieren" means "to happen" aka "vorkommen." However, from my experience they are not necessarily interchangeable. One hears "Was ist passiert?" for "What happened?" but I have never heard "Was ist vorgekommen?" "Vorkommen" is more frequently used in the sense "Das kommt nur selten vor," etc.
Because the subject of the sentence (in this case "es") has to follow the verb immediately. The word orders are very strict in German. Usually it is subject first followed immediately by the verb, but in this case, because the sentence starts with an adverb, the subject is pushed out of its normal position, the verb has to always be in the second position, so the subject is to follow in the third position immediately after the verb. I hope that answered your question.
And by the way, the correct term for thank you is "Vielen Dank".
I was translating this sentence from English to German, I got it wrong and the suggested correct translation was "Ansonsten kommt es eher selten vor."
I'm just wondering where the 'eher' came from? Does 'selten' have a different meaning to 'seldom' which necessitates that 'eher' be included? I know in English 'rather seldom' isn't really any different to just 'seldom' but I'm just curious whether it's the same for German.
FreekVerkerk: I agree with az-p. The sentence is correct in its meaning, but DL likes to go more word for word. In these cases, I usually try to second guess DL by trying to imagine what they want or are trying to practice, rather than focusing on reality. When I say "usually", that means that I forgot again this time, and wrote the same as you did ;-)