Don't forget there are two renewal dates, 30th September and 31st March. It could be that the main one started for the 31st March, which would make sense as it's almost the start of a financial year and the beginning of the summer season
After companies had delivered the holidays and recieved all payments, they then probably had the cash to be able to pay for the renewal in September/October. Everyone else would have likely followed suit.
Now as a result, people have naturally moved to September as the main date, possibly because people were booking holidays with a longer lead time, the CAA would have more funds earlier on to stand a better chance of paying off any collapses
Just my two pennies, but seems logical enough to make sense. More info on this Wiki page:
https://en.wikipedia.org/wiki/Air_Tr...s%27_Licensing