I doubt it has anything to do with the cold nights. It’s considerably colder up here in Michigan and my sequoia is doing just fine. I suspect your tree dried out too much sometime recently. There would’ve been a delay between when it got too dry and when it started turning brown.
An alternate possibility is that it has stayed too wet too long and has root rot, but I think that’s a much less likely scenario in your warm, dry Arizona climate, especially since sequoias really like to be very well watered. So, unless it’s in a very poorly-draining soil, I doubt you’d get root rot.