Hello Cash
Unfortunately it is becoming quite usual for assessment to take place up to a year before the end of the award. You are quite at liberty to ask at the DWP for their reason for early reassessment, though I have doubts as to whether they will explain to you - it really does seem to be normal procedure.
The best you can do is to gather together as much recent information as you can and send this along with your renewal application; follow the guides carefully to give yourself the best chance of the award you feel you should have.
If you have any issues at all, then get back to us; if you bookmark the page you should get back to us easily and give us the best chance of being able to help you straight away, without having to search for your previous mail. Good luck with the reassessment.