Reduction in SSI starting January 1, 2011

My father has been receiving SSI check for his disability over 10 years and would like to know why his SSI monthly check had been reduced starting January 1, 2011. This is the first time ever that his SSI monthly check has been reduced to almost half of the amount he used to receive. He harldy could live with the amount he received before the reduction, now is even harder for him to manage with rent, foods and personal items. Usually, he receives a letter stating that his SSI check would add to the monthly amount, but not in reduction. Does other Senior Citizens received reduction in SSI monthly check also or is this just in his case?

Thank you