Hi, Tim--my guess is that our unusually hot and dry summers have caused cumulative drought/ heat stress for your Sequoia.  Add to that the moisture competition from nearby Doug firs, and it makes it really hard for the Sequoia.  When a tree is stressed like that, it's very vulnerable to insect and other problems.  It's not a good sign that the top of the tree is turning brown--that almost always indicates a problem with the roots--too much or too little water, or a root fungal disease. I would strongly suggest that you contact an ISA Certified Arborist to get a firm diagnosis--and to see if there is anything you can do to save it.  You can find one near you here on the ISA "find an arborist" website https://www.treesaregood.org/findanarborist/findanarborist