An asteroid hit the Earth. Many species became extinct shortly thereafter. What other things happened just before and after the asteroid? Even the science is inconclusive.
Science often has to speculate, BUT it does so in an intelligent, systematic manner based on existing knowledge; asking "what if?" and seeing which possibilities best fit the available evidence or experimental results.
In the example cited, the end-Cretaceous extinction was one of a few major, and several smaller, mass-extinctions that have left evidence of their having taken place, throughout the span of life on Earth - currently some 500 million years.
As well as the asteroid impact, there had been co-incidentally been a long-term, major series of volcanic eruptions in India (and I think Siberia) sufficient to alter the climate for long enough to make conditions for the dinosaurs especially, very difficult. In this model, the asteroid impact's additional but short-term effect on the climate was the last straw for the poor creatures.
There was an even bigger mass-extinction millions of years earlier, in which climate conditions were exacerbated by most of the planet's land mass being agglomerated in one giant continent.
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Actually there is a flaw in the question: "Many things happen at the same time... singling one out as [causing] another..." Logically, the cause has to exist prior to the outcome even if the process becomes continuous by steady repetition of cause. It is therefore necessary to establish the events' relative chronology to establish which causes what.
