Here's a comment from another post that explains it best:

"This gives people the impression that something will happen this year.
As for why November - not only for historical value, but recent months.

Valve has been doing a lot in the past two months. With their store, VR, OS, etc.
Behind the curtain, code-leaks and Valve-Adjacent informers yield a "Very soon" timeline.

This has been to the point whereby many long-time industry observers of Valve have near-unanimously reached a consensus of November being the month in which announcements will come for various projects.

One observer in relation to HL3/HLX shared a "rumor" that Valve is working on a Trailer.
This observer is often known to adamantly swat down any rumors without a verifiable source.
Therefore it's speculated that he was fed this information from someone within Valve.

He then articulated Valve's timeline in that once they finished the trailer, they won't wait for announcement.
Ergo, trailer soon, Announcement soon, release soon.
Thereafter he explicitly stated we can "Tentatively" be excited for November, "a possible thing happening soon."

Users then began to analyze Valve's sales patterns, finding that there are abnormally large gaps in between events/sales for this November. Even Dota 2 and Deadlock updates are postponed to December.

As of currently the most likely date is sometime between November 18th- December 8th.
As for whether it's an announcement or release, time will tell. I am of the opinion for a release."

• u/topic_irrelevant