If one classifies the precipitation climate over the southwest U.S., there are four major regimes. The first (row a in the image below), covering most of California, northern Nevada, and the upper elevations of northern Utah and western Colorado, features a pronounced winter maximum. The second (row b), covering much of Arizona and portions of southern Utah, New Mexico, and southwest Colorado, features a pronounced maximum during the monsoon. The third (row c), covering the plains of eastern Colorado and New Mexico, is somewhat related and has a maximum in the summer with less precipitation in the winter. Finally, there is a fourth regime (row d) in the lower elevations of portions of Nevada, Utah, and western Colorado where precipitation peaks in the spring. This includes the Salt Lake Valley.
|Source: Steenburgh et al. (2013)|
Why October? I can speculate, but it really deserves some investigation. As suggested by David, there could be a magic point as the monsoon tails off and we begin to see mid-latitude storm systems coming in that enables more moisture and precipitation to move into this area. It might also be that this area got pounded by a small number of larger storms during the past few decades, skewing the stats for October (I haven't checked to evaluate the statistical significance of the October maximum).
In any event, eastern Utah has always been a little weird and even Mother Nature likes it that way.