Tree questions

We recently had two nice trees die.
One was a weeping mulberry, the other a desert willow.
Both were very hardy in our region.
The desert willow died because our leachfield, which it was planted on, failed, and it basically drowned.
The other, the weeping mulberry, was on sandy soil, but never was healthy. We have another in the front yard, a MONSTER, that we have pruned so the grandkids have a massive "fort" under the limbs. I would estimate shade diameter (not right term) at nearly 20'.
Today, I pulled the dead smaller weeping mulberry. The roots were stubby. I did not dig down to see if it hit a caliche shelf, being 103 here today. The roots did not run off shallow rooted, though, as I would suspect if it had hit a caliche shelf.
What would cause a tree to have roots that ended in nubs no further from the trunk than one foot, including one huge one that would have been the tap root?
Ideas appreciated.
We're going to plant a chaste tree, as they grow here very well, and have lots of flowers.
Steve
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"Chaste" ! You mean the poor thing will never be allowed to have, uh, YOU know!
HB
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