Pronunciation of 谁/誰
Quote from “谁”字注音规范化探讨（现代语文(语言研究版), Modern Chinese, 2007年11期 ）


  摘要："谁"在现实生活中应用十分广泛,可就是这样一个常用字,人们有时读"shuí",有时读"shéi",即使同一个人,在相同的语言环境中,对"谁"的读音也是模棱两......


This is all what I can see without an account.

And it suggests that even the same person cannot form a fixed pattern as when to use "shui2" or "shei2".

Maybe the full article can provide more information.

As a native, I can say that I don't know either. So the two pronunciations might just be two possible pronunciations of the same character, both widely used, and mostly interchangeable.