Whoops, I didn't notice that little blurb. On August 9'nth, he gave his notice to leave on August 10'nth.
He could have been offered a job with very little warning... perhaps a Pfizer-rank/Amgn type outfit saying we want you but on condition of starting right now.
Second, a health issue involving him or his family(sorry to speculate on something bad), but this does not make good sense since keeping one's insurance would seem important.
Three, he ran off somewhere, found the love of his life, wants to climb Mt. Everest ala mid-life crisis. I do hope it's option 3, for our sake, but the whole thing makes one quite uneasy.
Of course, the fourth possibility is some varient of the above with some flack or intra-Cortex battle about development plans. That would be possible.
Again, the time to abandon ship from an employee standpoint would seem to be when they were running out of all funds, not after they landed the funding, unless the other shoe is about to drop.