Start of the new school year, family plans, work issues, health issues (older crowd of players), and other issues that happen at last minute. Each player still believes turnout will be fine without them. It's sad when too many miss out on the same night.
This is the unintentional side imbalances. We don't know the reasons why people aren't showing up, we just know the results of people not showing up.
Here's a look at active pilots (i.e. those currently flying) over time for the 3 frames in August.

Here's a look at the ratio of Axis to Allied pilots over the same time period.

We can see that in frame 1, the Allies start with a roughly 1.2:1 disadvantage, which may be in line with the designer's desire to balance fighters. We see that the light colored frame 1 lines are closest. Combat is joined and the Allies lose most of their planes. At T+45, we see the Allies return in their previous numbers, then both sides dwindle off in numbers as the event progresses.
For frame 2, the Allies start with a slightly worse 1.3:1 disadvantage. Again, combat is joined and the Allies lose most of the planes. At T+45, we can see that some people have chosen to log rather than return. We don't know why, but it's obvious they are not doing what they did in frame 1.
For frame 3, the Allies start with a bad 2:1 disadvantage and as combat is joined, it quickly grows to a 9:1 disadvantage. Again, more people log rather than try a second life and of those that try a second life, the odds go quickly to 3:1 disadvantage and we lose more people who decide to log rather than return.
While we don't know for sure why people are choosing to log, it is clear (to me anyway) that Allied pilot behavior in frames 2 & 3 deviates from frame 1 and that choosing either not to participate at all or to fly only 1 or 2 sorties has a pretty drastic impact to those who fly the whole event. It's also clear to me that once the odds go beyond 3:1, it's pretty futile to launch and try to get anything reasonable done.
So, while there is unintentional imbalances with various sides meeting or slightly exceeding max commitments or not making minimum commitments, the more serious issue seems to be that the Allies were totally overwhelmed in each of the three frames.