I’ve run across some indications that there may have been a contagious disease in SW Utah at the time, leading to a number of people falling sick shortly after the wagon train went through. Possibly one of the people in the train was a carrier.
The locals promptly jumped to the conclusion that the wagon train must have poisoned the springs, somewhat as during the Middle Ages the Jews were accused of poisoning the wells and causing the plague. Hey, a Jew passed through town just before the plague struck, therefore...
Just a supposition.
That is quite possible, plus it seems that there were some watering holes that had bad water.