The 11,5000 year ago date originated because that was aprroximately the end of the Younger Dryas cold period and saw a rapid temperature rise occur in the northern polar regions (as shown by the Greenland Ice Cores). Because those writing popular "alternative history" books have little or no understanding of climatology they assume that such a temperature rise must have had a catastrophic origin and to have affected the entire planet. A pole shift (physical as opposed to magnetic) is then presented as the most likely explanation (they convenitently ignore the fact that if a shift such as they propose did occur, Greenland should have got colder, not warmer ... )
It's also used as the date for the Pleistocene/Holocene transition.
There is no evidence of any increase in animal mortality rates around that time. The megafaunal extinctions (most famously the mammoths) occurred over a long period and, in northern areas at least, appear to have been caused by increases in precipitation rates leading to changes in vegetation and competition from more efficent grazers such as bison and caribou - which saw their populations expand at the same time that horses and mammoths were in decline.
So, in answer to the question "why is the pole reversal so popular among doomsayers and conspiration theorists" the answer is because they can't be bothered to study the science and hard evidence that exists surrounding the termination of the last Glacial and instead jump from assumption to assumption with their feet never once touching reality .....
Oh, and a pole shift is also the only possible explanation for the millions of non existent flash frozen mammoths that haven't been found in the Siberia permafrost
