I've seen a client have an internal page just suddenly be de-indexed.  What appears to have happened is that Google saw it as a near duplicate of another page on their site, and dropped it from the index for that reason.  Then, magically, it reappeared a week later.

You may be seeing something like this here.  See what Moz Pro thinks in terms of duplicate content on your site, and if the home page gets called out along with another page.