A brilliant observation, Zod. Having spent a fair number of years in music retail in my younger years, I have a guess. My assumption is that the different record labels/distribution in each region try to spread releases out as evenly as possible over the 365 days to manage to a consistent revenue/income level throughout the year. To accomplish this they schedule release dates so that too many major artists don't release on the same date and have the consumer be forced to make a choice at the checkout counter and potentially cannibalize sales from one artist in favor of the other. Obviously there is a fair bit of "science" involved and it's not as simple as throwing a dart at a Tuesday (in the U.S.). They plan releases based on which artists have the biggest sales potential and gear the release dates/marketing strategies to times of year when consumers will have the most disposable income. If they didn't, then you'd see the 4th quarter (Christmas shopping season) heavily weighted with new releases since people are more apt to spend $, and bands would all want a part of that same pie. As it is, one can easily see when most of the big movies, games, CDs, etc. are released - all in time for Christmas.
Case in point: The Star Wars Trilogy video release, and video re-release, and re-re-release, and then DVD release and re-release, and special extended edition release, etc., etc. etc., have all occurred (to my memory) during Q4... A brilliant strategy by Lucasfilm/20th Century Fox which has netted them billions because a large percentage of the population loves Star Wars, and it's an easy thing to pick up and put in your shopping basket when looking for a last-minute gift or trying to pick up something as a gift for someone who is hard to buy for. The Star Wars blu-ray release has not been announced other than it will arrive in 2011, but it's widely speculated that it will be Q4.