Why did flight attendants need to be seated early landing at Palm Springs?
I can't say for sure, but here's one plausible reason: Palm Springs is in a valley and surrounded by much higher terrain on three sides, which requires more maneuvering to land than at other airports. You can see on the VFR sectional that the airport elevation is 476ft MSL, but there are mountains over 8000ft within 10 miles of the airport. If you check the instrument approach plates for runway 13R/31L (it's the longer one, so most likely to be used by an airline), you can see that the flight paths are far from straight:



RNAV 31L



RNAV 13R

I can imagine the pilot would brief the cabin crew to be seated well before landing for safety while the aircraft flies the approach. Mountains often produce turbulence and high winds too, which would be another good reason to be seated while maneuvering at a lower altitude over that terrain.