It could be A LOT of different things. If you were not expecting it, could be that they discovered a miscalculation and back paid you. This has been happening a lot lately, they've found that people were mis-paid hundreds or thousands of dollars.
Best thing to do would be to contact DFAS and ask about it. 1-800-321-1080
Remember, if it's an oversight, they will be taking it back, with or without warning.