I suspect it is a combination of temperature and water availability.

Thank you, I first thought that but wouldn't the leaves further from the root be the first to die?

Not really, the upper leaves tend to be younger and their surface layers are less 'tough' and more susceptible to wind/evaporation.

I would expect them both to be fine next year. Keep them well watered and that is a good watering can of each daily until we get some decent rain.