You've probably heard before that you should never defragment your SSD; conventional wisdom says not only do solid state drives not need defragging, doing so would cause unnecessary writes to the drive. Well, Windows does sometimes defragment SSDs — on purpose.
Scott Hanselman explains on his blog how Windows treats SSDs after digging into the topic with developers on the Windows storage team. Basically, if you have Volume Shadow Copy turned on (it's enabled when you have Windows System Restore turned on to be able to rollback to a previous system snapshot), defragmentation will happen once a month by default. Contrary to popular belief, SSDs do get fragmented, though not to the extent mechanical drives do.
Don't worry. Windows' defragmentation of your SSD is meant to prolong your drive's life and make it run optimally. For further reassurance and the full explanation, check out the link below.
The real and complete story: Does Windows defragment your SSD? [Scott Hanselman]