The voting system changed this year. All members of CAMRA were invited by an article on the front page of the January edition of What's Brewing to vote on line for their favourite beers, you could vote for up to five beers in each of the categories from a list of beers brewed in your region (Yorkshire & North East in my case). There were some options for those who "don't do computers" to have a vote.
I can't find out all the details about how the regional lists were assembled, ours is seven sides of A4 long but doesn't include every beer (just most of them). I think CAMRA's Brewery Liasion Officers had the biggest say in what beers were eligible under the rules and there was no scope for member nominations off list.
Mallinsons isn't on the 2012 regional list, but Magic Rock had a number of beers listed and I'm very surprised Curious or Cannonball aren't among the winners. Mallinsons probably could have been on it but may have fallen foul of the rules, the beers need to have been available for 3 months or more every year and the brewery must supply the OG rather than ABV to be eligible (I have no idea why the OG is required, it's supposedly something to do with how they are put into categories). Newer breweries were also excluded as the qualifying date was 1st October 2011.
After all members had their say, I believe the winners went to a regional panel who whittled it down then it was whittled down again on a national basis ending up at the GBBF final selection. The winter CBOB is slightly different as the winner is finally picked at the Winter Ales festival.
I think Hardknott have some beef with CAMRA and someone else refused to supply to the GBBF one year in a fit of pique at not being shortlisted for CBOB (NOT Brewdog!) other than that I think brewers like to be considered for awards.