the leader of the party with the most members in its caucus automatically becomes majority leader
Well, the question remains. How does the “leader” of the majority party become it’s “leader?”
The caucus for each party votes. It is an internal vote, so if Manchin doesn’t vote for Reid and votes for, say, Ben Nelson instead, Reid would still win the vote and become Majority Leader if there are at least 50 Senators in the Democrat caucus (including independents Lieberman and Sanders; Biden breaks the tie). It would be far different if the vote was on the floor and Manchin was the 50th Democrat, where not voting for Reid would allow McCollum to win 50-49.