Answer:
Deforestation.
Step-by-step explanation:
The source of the logs changed since trees don't grow faster than humans. When people build a big old mall like Chaco Canyon all of the trees are used for it. When it's done, the natives either wait for new trees to grow (which takes a few generations) or use different trees! Not as strong, but they'll do the trick. After those trees are gone (and there's not a lot of forests in NM, I've been there) the natives go hungry. And when people are hungry, they get mad. At others. So war broke out, and the wise natives who lived there decided to go. By the way, the natives chose to use different trees. So, different trees (to make logs) means a different source of logs.
Also, A young juniper tree and a old juniper tree are still juniper trees, so the source is the same. Hope this helps!