"... had to offload pax after the pilot requested extra fuel because of bad weather. I don't think there's any great mystery here after you look at the flightaware map, and consider altitude/TO temps."

We have a winner.

IFR flight planning has a lot of rules & regulations, exceptions, and local conditions the dispatchers must follow. The forecast, not necessarily actual, weather conditions at the time of arrival is the first factor. Next factor is the forecast at a suitable alternate. "Suitable" depends on a lot of things. There must be enough extra fuel to satisfy the regulatory minimum for those particular airports, that particular flight, and that particular day. Next, add in any local conditions at SLC (weather, airport altitude, temperature, etc.) This results in extra fuel (weight) to legally make the flight that day.

Extra weight means something else must be offloaded, either passengers, baggage, or cargo. What gets cut is strictly an economic decision. In this case, it was the passengers.

(And, like almost every other response on this thread, this is only my speculation, not an official reason)