An alternative theory for the decline in August passenger numbers
Compared to last year there's the loss of Monarch, as previously mentioned.
Another factor that affected only August is that Ryanair operated 7.9% fewer movements than in August 2014.
The number of RYR movements in August 2015 was roughly equal to the number operated in July 2015, but in August 2014 and in August 2013 there was a peak in flights (about 4 legs per day). In August 2015 this extra peak didn't happen.