Most likely to be crosstalk.
Have a read of this http://www.increasebroadbandspeed.co.uk/2014/vectoring-crosstalk-crisis
Good to see others are allowed to post that link
Don't know whether I am or not, I've posted it a couple of times and don't think it has been deleted. Have you had it deleted?
It could just be as simple as adding another customer on a pair that particularly impacts your pair. My max rate has dropped from about 60M when I was about the first in the cable to about 46M now everybody has Infinity in our road.