Freddie Mac announced this week it is closing down its mortgage securities trading operation in order to focus more on its core housing mission. But observers contend there also are a number of non-mission related reasons for shuttering the GSE’s infamous Securities Sales & Trading Group. Freddie officials released a statement early this week that said the firm was ceasing the market-making activities of its Securities Sales & Trading Group [SS&TG] and transferring the group’s mortgage…