Well, the missing 'E' in the B series is what made me say there's no pattern.
If you ignore that (everyone has a 'flyer' now and then), the pattern is to omit G, J, K, O, Q, R, V, W, and X presumably due to possible confusion of:
G, O and Q with D,
J with I,
R with P,
V and W with U and
K and X with each other, perhaps.
Maybe the real screwup is the inclusion of 'E' in the A series - it can be confused with F pretty easily and they just didn't realize that until it was too late. So, there won't be a CE, and the future codes will be:
2008 - CC
2009 - CD
2010 - CF
2011 - CH
2012 - CI
2013 - CL
2014 - CM
2015 - CN
2016 - CP
2017 - CS
2018 - CT
2019 - CU
2020 - CZ
Notice my optimism regarding the future. It's here that the CL's jump in and opine that the current Democrat political agenda is to prevent that from happening.