所有 = "every"
事情 = "thing"
So, "every" needs to be joined together with "thing" to make "everything"
Similarly, 所有 needs to be together with 事情 to make 所有事情 = "everything or all things"
So, if you separate them and say, 所有发生, it means "every happenings", and not "everything" as in 所有事情. Thus, saying 所有发生 would no longer fit in with 事情 to mean "everything"
Let's take @Pedroski's examples which on the face of it appear to contradict the above. But, not so. Let me explain.
(1) 所有发生的这一切都不仅仅是巧合 // (2) 告诉我你身边的所有发生的一切
Here, though it may "sound" acceptable to some Chinese ears, there is in fact a Pleonasm Redundancy, meaning the use of more words than are necessary to convey meaning.
所有 = "every / all" and 的这一切 / 的一切 = "all of / every", (both having similar semantic content, thus a Pleonasm Redundancy)
Sometimes redundancies which violate grammatical niceties are "acceptable" for reasons of emotive emphasis or stylistic characterization.
请给我描述一下所有发生的事情 is semantically different from 今年发生的所有事情
In the former, the 所有 must be read together with 描述一下, (i.e., 描述一下所有, meaning "describe completely"), i.e "do not leave out anything"
Thus the sentence, 请给我描述一下所有发生的事情 reads "Please do describe completely to me, (请给我描述一下所有), what were the things that happened, (发生的事情)
In the latter, 今年发生的所有事情, the 所有事情 simply means "Everything"