The Heroes of the Storm patch released in mid-August made a lot of changes to the game, including limiting the maximum Hero League party size to four players. Now that number has been cut even further, as Blizzard has decided, based on player feedback and its own data analysis, to restrict Hero League queues to a maximum of two players.
Blizzard's internal data indicates that there are often "large skill gaps" between players in Hero League parties of three or four. That's not a problem in itself, as the players have presumably agreed to work together despite the imbalance, but it can become problematic when the matchmaker completes the team by pulling in a fifth player who queued alone, and doesn't necessarily want to be saddled with noobs.
"This can also create situations in which party members will discuss strategy with each other, but don’t end up communicating the game plan to their fifth teammate," Blizzard explained. "On the other end of the spectrum, solo players may disagree with the party’s decisions, or show unwillingness to help the team. This can result in a friction for both sides if communication doesn’t improve."
Simplifying the decision to reduce the limit even further is the fact that players queue for Hero League in parties of three and four "far less often" that soloists or duos. "As such we feel that large parties represent a small enough portion of Ranked games that we’re comfortable removing them from Hero League in order to improve overall match quality," Blizzard wrote.
The new queue size limit will be implemented in the next Heroes of the Storm update, but it won't necessarily be the end of the matter. Blizzard said it will monitor how the new rule affects the game, and will continue to tweak it as necessary.