It's a good question and I think in the right forum. I'll add it to 'plants' and 'trees' also though.
It makes me wonder about the large bronze fennel I have growing on the garden fence. No trees near by or other vegetation for that matter so I don't know if it's causing any problems or not. It seems like I remember reading in one of those 'companion planting guides' that fennel didn't get along with anything.
EDIT to add a quote...
Fennel Companion Planting. Fennel is a highly aromatic and slightly odd plant that is pretty much a poor companion plant for everything. Fennel companion planting generally only works when growing dill. However, even dill is a poor choice because the two herbs tend to cross-pollinate.
I have not found any references to fennel being severely allopathic to any plants.
The lack of worms might be an indicator of lacking soil biology, particularly fungi which both of your trees needed to be healthy and long lived.
Have you inspected the tree trunks for small holes in the bark? This is an indicator of borers being the culprits of their death.
If there are none, then you might consider taking soil samples to a lab for a biological tests as well as a mineral test. That might give more clues as to what went wrong.
One other thing to consider is voles or gophers, they both leave tell tale signs if they are there.
I do know from experience that other vegetables seem to show that fennel can be mildly allopathic to them but I have two pear trees and two plum trees that I grow both fennel and garlic around with out any ill effects (three years of doing this).
The avocado might have gotten sunburn (I had this happen to a year old tree and it died from it).
Root disturbance is something that can also wreak havoc to young or old trees, so if there was any of this going on, that may have contributed to their demise.