What happened is not all that uncommon. I would suspect that the most likely cause was hot brakes, as AAR-90 pointed out. Also, depending on the aircraft's ground turn time and SLC air temps, the brakes may have been hot already and didn't get sufficient time to cool.
Just to give an idea of the speed restrictions regarding the landing gear for an aircraft Other than the 757:
The max speed on the DC-9 to extend the gear is 300kts
The retract speed is 250kts in our manual.
The max tire speed is 195 kts.
On airlines that operate jets on multiple hops to short fields, they often extend the gear for an extended period of time to cool the brakes. An example is Air Mike's 727s--lots of short hops to small island airstrips where this happens a lot.