Here's one I took today:

Mine do best with a lot of water. Dig down and see how dry it is, or you might find it to wet/water logged.

My first thought was that it looks very, very dry. Yes, these are desert palms BUT, in nature they rely on water from underground aquifers. Increase watering.

They've been fine for two years, as have all my other palms. When the first one died in July, I cut it down, and it stunk of rot.
The trunk was very wet, so I just don't think it's a lack of water.
And I don't think watering every four days is too much...it never has been before. That's why I think something else is going on.

Then check to see if the drainage is bad, they could be water logged like I said before. You will have to dig down into the dirt to find your answer.

Ironically, the effects of being dehydrated and being soaked look the same. Dehydrated plants are not getting enough water and soaked plants have rotted roots that cannot utilize that water.

Worst case scenario (for me), is someone is poisoning them. Is there anything someone could put on these plants that could kill them like that...?
I've never had any problems like this before, and these Palms were growing phenomenally well.. Beautiful all green fronds, no yellowing. Then bang...just like that, one failed. Now another one.

Did you dig down to see what is going on at the root level?

I planted a new palm right beside the one that had died. When I dug the hole for it, the soil was not wet.
The affected one now, I dug down 6 or 8 inches two days after watering. Not wet.
The soil in that area drains pretty good.

A palm grower near me had a mysterious die off of some of her palms, and it turned out it was an underground gas leak (too about half a year to figure that one out). Certain substances can also leak into the soils and cause death as well (oils most likely).

Could be phytophthora. Don't water them overhead if you usually do that. Also, water away from the trunk, maintaining a dry space of 4-6 inches around the base.