'Occur' is a better word because 'occur' is more detached and scientific than 'happen', and it also gives more of a suggestion of a causal link. We often use 'happen' for random incidents. The origin of the word 'happen' is to do with luck and chance. This is still evident in words such as 'haphazard', 'happenstance' . Although this earlier meaning has been lost, and 'occur' and 'happen' are now near-synonyms, we still tend to use 'happen' for unpredictable and chance events. Accidents happen. Nowadays, we also use 'happen' for organised events. For example, 'the Olympic Games happen every four years'. We don't tend to say 'the Olympic Games occur every four years'. This suggests that there is some kind of scientific reason. Solar eclipses occur very rarely - this is more detached and technical. 'Occur' is the better word when there is neither pure chance nor human invention involved.

No difference in meaning, the Author probably wanted it to sound more high brow. happen is used more in common everyday situations, occur used less often and mainly to sound more academic/posh/educated/etc...

They mean the same thing but there are differences. Unless you're experienced in the English language it doesn't really matter too much. -------------------------- I would use occur in the following contexts: Formal conversations; When a time or place of an event is specific; Expected events, like dates: The Spring Bank Holiday that would usually have occurred at the end of May; Discoveries, ideas: The discovery occurred in Mono Lake, California; ...and happen in the following contexts: Informal context; Rough time or place; Random events, usually without being planned; http://ell.stackexchange.com/questions/1135/difference-between-to-occur-and-to-happen

occur 比较正式的用语,可用于具体或抽象的事物,通常指按计划或规律在较为确定的时间“发生”的事,在表示具体的事物时,可与happen换用. 如：I'm afraid that this would occur during my absence. = I'm afraid that this would happen during my absence. 恐怕这事会在我不在时发生. 但是,如果表示“某想法”出现在人的头脑之中,这时不能用happen代替. Dose it ever occur to you that I am sometimes thinking? 我时不时想的那件事你是不是也曾再你头脑中出现过? happen常指具体事件的发生,特别指那些偶然的或未能预见的“发生”. 比如： New things are happening all around us.我们身边总有新事发生. Happen还可表示“碰巧；恰好”之意. 如：It happened that I had no money on me.我刚好没钱了