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The decrease has been explained as the end of the honeymoon for BioWare's space-age MMO. A large number of casual and trial players signed up in the early days, and have since cycled out again, meaning this number may be more representative of the longer-term player base.
EA also explains that this number was within the expectation that the companies had when BioWare was acquired by the publisher back in 2007. Despite the drop in numbers, The Old Republic is still profitable, and settled happily in its #2 spot in the MMO landscape.
The news was revealed in EA's latest financial report, which also suggested that there's a bunch of new PVP content and guild updates due out by the end of the year, as well as a new, major patch titled Allies (being a financial report however, there wasn't much information on the actual content of these new bits and pieces).
For the full report, head to EA's official website, where it's hosted as a PDF.