Is this a function of the ladder, or just coincidence?
It's a logical consequence of two functions of the ladder.
1) Games will be matched up against opponents that are close to each other.
2) Aside from the first four games, everyone will get a game every 3 days.
(Invisible 3: Games don't have to be finished [changing the rating] for new games to be made)
Because everyone on the ladder is open for a game at the same time, the ladder is able to freely match you to the closest opponent possible. Since not everyone can finish a match within 3 days, it's likely that they'll be near the same position they were at last time. Being near the same people every time creates this "block" of common games.
For example, lets say we have this random part of the ladder: ... A, B, C, D, E, F, ...
Assume that A-F are really slow players and/or are stalling on purpose. Also assume that A-F have not played each other yet.
The ladder might match A/B, C/D, E/F.
In 3 days, it matches A/C, B/E, D/F.
Another 3 days, it matches A/D, B/F, C/E.
3 more? A/F, B/C, D/E.
Finishing it off in 3 more days: A/E, B/D, C/F.
Obviously not everyone is as slow as A-F, but due to the force-end and the fact that it's multi-day, it's not hard to envision that 3-4 players stay close enough to create a group of common games.
Edited 5/5/2014 20:35:35