Well, based on what's written here, the only assumption I can do is that the temperature difference between full sun during the day, and cool temperatures during the night, might have stressed them out too much. 20 degrees C weather over here, means that clay pots can be 30 degrees C if they are in full sun. During the night, winds can help evaporation take place, which drops the temperature in the pots a few degrees C lower than the actual air.
In the shade it might be less of a difference. Because shaded parts usually get less sun, and less wind: less temperature variation.
But it shouldn't happen with temperature ranges like that, just a few degrees C. I've kept tropical plants outdoors at 2 degrees C for a week, and they're still alive and kicking after being placed in 30 degrees C. I'm asking for images because generally, plants tell a story through their foliage and trunks. It's easier to find a root cause with a story of the past. If the foliage stayed green, then it could be a sudden death. Especially in tropicals, and this would give us a lead to work with. If they turned color before the plant died, then it might be something else. From the color and progression of discoloration, we can tell a lot. As a matter of fact, I am so sure about this, that I once challenged my team to beat me in diagnostics of plant issues. They lost, 14 to 1. The 1 was an issue I had never seen before; iron uptake was blocked due to an overdose of antibiotics. The diagnosis was correct, but the team beat me with experience about the cause; they had seen this before and I hadn't. I figured iron was either not supplied, or blocked due to the medium having a pH that was too alkaline. It turned out the antibiotic blocked an iron pathway in both bacteria and fungi, as well as in plants. We had to switch types.
Summarized, all I know now is that plants in the sun died and plants in the shade lived, and that general temperatures are a little lower than usual.
Another possibility is, you're one of the few people in the world using compost. When temperatures get high enough for composting micro-organisms (MO's) to become active, but the plants stay dormant.. The MO's could cause issues because they'll start eating the tree that can't defend itself (trees don't fight very well in the cold, but bacteria do!). If that happened, the roots will be soft and the bark of the roots will come off easily and slimey. But since that's part of the natural break down process as well, it should be checked first before running other options. Otherwise you can't safely tell if it was natural because it was a dead tree degrading, or that it happened because of something actively feeding off of the live plant and killing it.
Things we can confirm or rule out with visual cues:
- Pathogens - possibility, shown on trunk and or foliage.
- Temperature issues - possibility, shown on foliage and general state of the entire plant.
- Watering issues - possibility, shown on foliage.
- pH or nutrient issues - unlikely, shown on foliage.
- Salt buildup issues - can be a cause, in the sun you have to water more often, shown on trunk and roots and foliage.
- Disease - viral, possibility, shown on the foliage.
- Other factors like animal damage or dogs peeing all over the plant - shown in foliage, roots or trunk.
- Antibiotic over use - unlikely because you're not the type.
- Other things I can't think of right now.
I'm working on the document on how these cues work as we speak. But you can assume it's going to take another week or two before it's finished.